It's been nine seasons since Gruden last coached. The success rate of head coaches hired after a 9+ season span between top NFL jobs has been a mixed bag. Most recently, Pete Carroll and Chan Gailey were hired in 2010 by the Seahawks and Bills respectively - each had 11 year spans between NFL head coaching jobs. Since his return, Carroll has a 79-48 record and a Super Bowl win. Gailey had a 16-32 record during his three years with the Bills.

Jon Gruden was previously the Raiders head coach for four seasons from 1998-2001. While there, he had a 64-38 record and a 2-2 record in the playoffs.

Including his seven seasons with the Buccaneers, Gruden has an overall record of 95-81 as a head coach, with five playoff appearances (5-4 record in playoffs). He had six winning seasons - two with the Raiders, and four in Tampa Bay.

Gruden was traded from the Raiders to the Buccaneers for two first round and two second round draft picks. That following season, he led the Buccaneers to a victory over the Raiders in Super Bowl XXXVII.

Gruden joins the Raiders on what likely will be a 10-year, $100 million contract, NFL Network Insider Ian Rapoport reported Friday. The pact is believed to be the richest coaching contract in NFL history, according to Rapoport. NFL teams do not disclose financial details on coaching deals.