The Lions announced the deal on their website and official Twitter feed.
After spending his first nine seasons with the Bears, Idonije jumped to Chicago's NFC North rival. He signed with the Bears' practice squad as a rookie late in the 2003 season after initially entering the league as an undrafted rookie free agent with the Cleveland Browns.
Primarily a special-teams standout and reserve defensive lineman for his first six full seasons, Idonije emerged as one of the Bears' starting defensive ends in 2010 and went on to post a career-high eight sacks. He combined for 12.5 sacks over the past two seasons, splitting time between end and defensive tackle in Chicago's nickel package.
Bears general manager Phil Emery publicly stated at the NFL combine in February that the team wanted to re-sign Idonije, but at a reduced price. The 32-year-old defensive lineman earned $2.5 million last season.
But as the offseason wore on, it became apparent the Bears were prepared to move on without Idonije after the club signed a handful of veteran defensive linemen to league-minimum one-year deals.