The five-time NFL MVP and postseason regular Peyton Manning said Monday he was retiring because, "after 18 years, it's time."

A month after his second Super Bowl championship and a parade through downtown Denver attended by more than a million fans, the soon-to-be 40-year-old quarterback thanked his family, his supporters, and his teammates and rivals. "Thank you for what you've done for this old quarterback."

Manning says he has no idea what he'll do in retirement. Citing new opportunities on and off the field, he said, "Life is not shrinking for me."

The tributes came pouring in. Broncos president Joe Ellis said, "Peyton Manning made our team better, he made our organization better, and he made our community better." General Manager John Elway added, "On behalf of all our fans, thank you."

Tom Brady tweeted his congratulations telling Manning, "You changed the game forever and made everyone around you better."

Manning's college coach, Phillip Fulmer, called it "almost magical" what Manning was able to accomplish in his four years in Denver following neck fusion surgery that threatened to short-circuit his career.

Instead, Manning went 50-15 in Denver, winning his fifth MVP award, surpassing Brett Favre's records for career yards, touchdowns and victories while leading the Broncos to two Super Bowls.

"I've said this a lot of times to people, he didn't get better each game. He got better every meeting, every practice, every period in practice," Fulmer said. "He was the most incredible player that I've ever seen of taking it from the film room to the practice field and then from the practice field to the game."

The knock on Manning was that he didn't win more Super Bowls. For a guy with such a great regular season record (186-79), his playoff mark of just 14-13 paled in comparison.

"People don't realize, it's hard to always be the best player," Fulmer argued. "For almost all his years, he had to be the best player. At Indianapolis, he never had a great defense around him. In my opinion, if he had been on teams like this last Broncos team -- a team not even as good on defense -- the guy might have six or seven Super Bowls."

Commissioner Roger Goodell called Manning "a great representative of the NFL both on the field and in his community," adding, "We are forever grateful for Peyton's unmatched contributions to the game and know his success will continue in the next phase of his life."

There are plenty of possibilities -- and advice -- to stay in football, from the broadcast booth to the front office.

Some believe he'll eventually be an NFL owner.

ESPN, FOX, NBC and CBS all said they would love to talk to him if he decides to go into broadcasting.

Just shy of 40, Manning will forgo $19 million and a 19th season in the NFL, where he served as both a throwback and a transformer during a glittering career bookmarked by an unprecedented five MVP awards and dozens of passing records.

"Peyton was a player that guys wanted to play with," Elway said. "That made us better as a team and I'm thrilled that we were able to win a championship in his final year."

Manning leaves the league as its career passing leader and winningest starting quarterback, the only one in NFL history to win Super Bowls with two franchises.

His first came in 2007 with the Indianapolis Colts, who drafted him No. 1 overall in 1998. The Colts gave up on him after a series of neck surgeries forced Manning to miss all of the 2011 season and left him without feeling in the fingertips of his right hand.

A rare superstar quarterback on the open market in 2012, Manning resettled in Denver, where he threw 140 of his NFL-high 539 TD passes, including a record 55 in 2013.

"I get asked a lot about my legacy," Manning said before the Super Bowl. "For me, it's being a good teammate, having the respect of my teammates, having the respect of the coaches and players. That's important to me. I am not taking this for granted. I just love football."

The 18th season for No. 18 was by far his most trying on the field. He had to adjust to new coach Gary Kubiak's run-based offense and to unrelenting health issues on his way to winning his second Super Bowl. Off the field, he faced questions about his character, too.

Manning, whose dry wit and star power has made him a staple of commercials and late-night television for nearly two decades, had his squeaky-clean image take a beating as the final pages were flipped on his storied career.

The NFL is investigating allegations that human growth hormone was shipped to his home in his wife's name following an Al Jazeera report -- Manning dismissed the story as "garbage." And in a new lawsuit filed last month claiming a hostile work environment for women at the University of Tennessee, Manning was cited for his alleged harassment of a female trainer in 1996.

A torn ligament in his left foot hampered Manning all the way back to August. It led to his worst statistical season and sidelined him for six weeks before that fairy tale finish in Santa Clara, California, when his defense carried him across the finish line.

Constantly harassed, never quite comfortable -- sort of the way the whole season played out -- Manning walked away with his second NFL title after Denver's defense, with seven sacks and four takeaways, all but handed him the Lombardi Trophy in a 24-10 victory over the Carolina Panthers.

"He had to do several things different this year," said his dad, Archie, a former star quarterback himself. "Had to take off during the season, which he'd never done before. He ran the scout team, which I don't think he'd ever done, and he dressed out as a backup, which he'd never done."

Manning also had to play the role of game manager for the first time during Denver's defense-fueled run to the title. "I'm just glad I was on the same team as our defense," he said.

Manning's retirement leaves the Broncos with $19 million in salary cap relief they can use to try to sign his long-time backup, Brock Osweiler, who's set to become a free agent Wednesday.

Manning finished in a tie with Favre for most regular-season wins with 186. His victory in Super Bowl 50 was his 14th in the postseason, one more than Favre, making him the NFL's only 200-win quarterback.

"There's no question that his work ethic is what made him into one of the great quarterbacks of all time," Elway said. "All the film study Peyton did and the process that he went through with game planning and understanding what the other teams did was second to none."

The Associated Press contributed to this report.