Arians confirmed his decision to reporters on Monday after ESPN's Adam Schefter reported that the head coach would walk away on his own terms:
"I probably didn't truly know until that kick went through that I was gonna retire," Arians told reporters after Seattle missed a potential game-winning field goal on December 31 in a 26-24 Arizona victory. "Everybody was speculating. ... Been an unbelievable journey."
Arians further discussed the decision on The Athletic, saying he wasn't enjoying the game the way he used to.
The 65-year-old leaves having compiled a 49-30-1 record in 80 regular-season games in Arizona.
The Cardinals improved immediately upon Arians' arrival in 2013. The team's record went from 5-11 in 2012 under Ken Whisenhunt to 10-6, and Arizona missed out on the playoffs by one game.
Arians built a reputation as a quarterback guru, and he more than justified the moniker through his work with Carson Palmer.
In his final year with the Oakland Raiders in 2012, Palmer had 4,018 yards, 22 touchdowns and 14 interceptions. He averaged 267.9 yards per game and had an 85.3 quarterback rating.
Over his first three seasons with the Cardinals, Palmer averaged 278.2 yards per game and boasted a 94.2 quarterback rating. At 36 years old, Palmer threw for a career-high 4,671 yards and 35 touchdowns in 2015, while his 11 interceptions were his fewest in a season in which he played in at least 10 games.
Palmer's 2015 resurgence coincided with Arizona winning a franchise-record 13 regular-season games. The year ended in disappointing fashion, though, as the Cardinals lost 49-15 to the Carolina Panthers in the NFC Championship Game.
Since that defeat, the Cardinals have taken steps backward as Palmer succumbed to the aging curve. Arizona went 7-8-1 in 2016 and finished 8-8 in 2017.
An injury to David Johnson compounded the slow start. The All-Pro running back underwent surgery in September after suffering a wrist injury in Week 1. Then, Palmer broke his left arm in October, which left Drew Stanton and Blaine Gabbert as Arizona's two best starting options at quarterback.
Arians' decision to resign before his contact expired after the 2018 season doesn't come as a major surprise.
The Cardinals are in a period of transition. A number of the team's core players are in or near the twilight of their NFL careers. Palmer turned 38 in December, and Larry Fitzgerald celebrated his 34th birthday in August. On defense, Karlos Dansby, Frostee Rucker and Antoine Bethea are all in their mid-30s.
Not only is Arians 65, he also revealed he had kidney surgery in February after doctors found renal cell carcinoma during an ultrasound. Arians also experienced a bout of diverticulitis during the 2016 preseason.
Ideally, Arians would stick around to help groom Palmer's replacement. Jared Goff's performance under Los Angeles Rams head coach Sean McVay compared to former coach Jeff Fisher is a testament to how the right coach is instrumental to a young passer's progression.
That plan hinged on Arians wanting to stay while coaching what's likely to be a losing team.
Given both his age and health concerns, Arians' aversion to staying on for a multiyear rebuild is understandable, but it leaves the Cardinals in a difficult spot this offseason. In addition to identifying a potential successor for Palmer, Arizona will need to find one for Arians as well.