DETROIT -- Matthew Stafford has agreed to a three-year contract extension that will keep him a Lion through 2017 and make him - for the second time in his short career - one of the highest-paid players in the NFL, according to a person with knowledge of the situation.

Stafford's new deal is worth $76 million over five years, includes $43 million guaranteed and gives both the team some downside protection in case he doesn't reach his massive potential and him a chance at a third mega deal while he's still in the prime of his career.

He was scheduled to make $12.5 million this season and carry a $20.8-million cap charge.

Just 25, Stafford already owns team single-season records for passing yards, touchdowns, completions and attempts, and he should break Bobby Layne's career passing mark this fall.

He has thrown for more than 10,000 yards over the last two years, and joined Tom Brady, Drew Brees and Dan Marino as the only quarterbacks to top 5,000 yards in a single season in NFL history.

Stafford said several times this off-season that he hoped to remain a Lion for the balance of his career, and the Lions had no hold-up handing over their second sizable contract in the past 16 months to a player who has overcome early injury woes to take a prime leadership role in the locker room.

Limited to just 13 games his first two seasons because of knee and shoulder injuries, Stafford has started every game the last two years and led the Lions to the playoffs in 2011.

After the Lions missed the postseason and his production waned last year - he threw 21 fewer touchdowns and completed just 59.8 percent of his passes - Stafford spent the entire off-season in Detroit.

"He's evolved into a leader," Lions president Tom Lewand said last month. "Now when rookies walk into the locker room, they see a quarterback who's been here four years and they see a different guy who occupies a different spot on the team, and a lot of that is because of his own efforts.

"I don't think we have a guy on our roster who's worked harder than he has, and I'm not sure that you'll find a guy who cares more than he does about this team and about winning and about his teammates."

Lewand did not return phone calls seeking comment today.

The No. 1 overall pick out of Georgia in 2009, Stafford is the fifth quarterback to agree to an extension this off-season.

Baltimore's Joe Flacco got $51 million guaranteed on a $120-million deal a month after winning the Super Bowl, Dallas' Tony Romo got $55 million guaranteed on a seven-year, $119-million deal April 1, Green Bay's Aaron Rodgers got $54 million guaranteed on a $130-million deal four weeks later, and Brady got $57 million guaranteed and a $30 million signing bonus on a three-year extension in February.

Atlanta's Matt Ryan also is expected to sign a new deal before the start of the season.

Stafford, who made $50.5 million in his first four seasons, Romo and Ryan are all represented by Creative Artists Agency, and Stafford's deal will give the Lions some cap flexibility going forward.

With Stafford and Calvin Johnson, who signed an eight-year, $131-million deal last March, under contract, the Lions can shift their focus this off-season to defensive tackle Ndamukong Suh.

Suh's rookie deal voids after the 2014 season, and his cap charge next year of $21.4 million makes it unlikely the Lions could use the franchise tag to retain him. Tagging Suh for one year would cost the Lions $25.7 million.

Suh said in late May he was "two years away from even considering" an extension but that he'd "love to be here in Detroit."

A couple weeks later, Stafford insisted that his contract wouldn't be a distraction this year, regardless of whether it got done.

"I don't play this game to get contracts," Stafford said at the time. "I play this game to win games, and that's the way I've always felt about it."