Adam Schefter says the Vikings will likely sign Kirk Cousins to a fully guaranteed three-year, $84 million deal, but it isn't official yet. (1:14)

Former Washington Redskins quarterback Kirk Cousins plans to sign a three-year, fully guaranteed $84 million contract with the Minnesota Vikings on Thursday, sources tell ESPN's Adam Schefter.

The landmark deal will make Cousins the highest-paid quarterback in the league.

"No final decisions have been made," Cousins' agent, Mike McCartney, said Tuesday. "The plan is to visit Minnesota first. That's all we can say right now."

Seattle Seahawks wide receiver Doug Baldwin tweeted that Cousins is a "hero" to other young players for getting a fully guaranteed contract.

Kirk Cousins is a hero for all the young players that will follow after him. Now we need more players to bet on themselves until fully guaranteed contracts are the norm and not the exception. — Doug Baldwin Jr (@DougBaldwinJr) March 13, 2018

Cousins, 29, has been one of the NFL's most productive quarterbacks since becoming Washington's full-time starter three years ago. During that time, he has ranked fourth in passing yards, sixth in passer rating, seventh in Total QBR and eighth in touchdown passes.

In 2017, Cousins topped the 4,000-yard mark for the third straight season. He finished with 4,093 passing yards, 27 touchdowns and 13 interceptions -- and a Total QBR of 52.3, his lowest as the full-time starter.

The 2012 fourth-round pick went from the backup to Robert Griffin III -- starting nine games in his first three seasons -- to a productive starter during his time in Washington. In 2015, his first year as the starter, he helped lead the Redskins to the NFC East title by throwing 23 touchdowns to three interceptions over the final 10 games -- starting with the "You like that?!" comeback win over the Tampa Bay Buccaneers.

He finished that season with 29 touchdowns, 11 interceptions and a franchise-record 4,166 yards.

After that season, Washington and Cousins couldn't agree on a long-term deal, so the team placed the franchise tag on him. Cousins responded by again breaking the franchise record with 4,917 passing yards to go along with 25 touchdowns and 12 interceptions and a Pro Bowl berth. But Washington missed the playoffs by losing their regular-season finale at home to the New York Giants.

That led to yet another franchise tag, as Cousins told the Redskins he wasn't ready to commit long term and wanted to gauge the direction of the franchise. It was clear by season's end in 2017 that both sides wanted to avoid another one-year tag. Rather than negotiate a final time with Cousins, Washington traded for Alex Smith on Jan. 30, agreeing to a four-year extension with the veteran.

Cousins heads to Minnesota, where the Vikings are coming off a 13-3 regular season with an appearance in the NFC title game. Last year's starter, Case Keenum, the quarterback who stepped in for an injured Sam Bradford early in the 2017 season, is expected to sign with the Denver Broncos when free agency officially opens Wednesday afternoon.

Along with Bradford, Teddy Bridgewater is also a free agent after spending eight weeks as the Vikings' No. 2 quarterback. He was deactivated for the playoffs in favor of Bradford.

Cousins inherits a roster built around the league's top-ranked defense and a plethora of weapons on offense. Adam Thielen and Stefon Diggs return for another season in which they'll vie to be the NFL's top receiving duo. The backfield also could become one of the league's best behind the return of Dalvin Cook, who is recovering from a torn ACL, and Latavius Murray.

Cousins' potential signing with Vikings moved Minnesota's Super Bowl LIII odds from 16-1 to 12-1 (tied with Green Bay for fourth best) at the Westgate Las Vegas SuperBook.

New England is the favorite at 5-1, with Pittsburgh and Philadelphia next at 7-1.

ESPN Vikings reporter Courtney Cronin contributed to this report.