Trades cannot become official until Friday, the first day of the league year.
Bly, a two-time Pro Bowl performer who earned a Super Bowl ring as a member of the 1999 St. Louis Rams, did not fit well in the Tampa 2 defensive scheme employed by second-year head coach Rod Marinelli and new defensive coordinator Joe Barry. Thus the Lions granted Bly and his agent, Kennard McGuire, permission to seek a trade two weeks ago.
Neither Bly nor McGuire was available for comment.
In Denver, Bly will join Champ Bailey in forming one of the game's top cornerback tandems. The Broncos needed a replacement for Darrent Williams, who was killed in a drive-by shooting after a New Year's Eve party in Denver.
Bly is considered one of the game's best big-play cornerbacks. In eight seasons he has intercepted 31 passes, forced 17 fumbles, and scored eight touchdowns (five interception returns, two fumble returns, one punt return). Last season Bly tied for the team lead with three interceptions and led the Lions with 18 passes defended and four forced fumbles.
Bly has one year remaining on the $22 million free-agent contract he signed in 2003 and the Broncos are expected to begin working on a long-term agreement with McGuire.
Bell, 25, became a starter for the first time last season, his third, and led the Broncos with 1,025 rushing yards. In three seasons with the Broncos he has rushed for 2,342 yards and 13 touchdowns and owns a career yards-per-carry average of 4.9. The speedy Bell provides depth and insurance for the Lions in the event top back Kevin Jones' foot injury keeps him sidelined into next season.
Foster started 45 out of a possible 48 games the past three seasons at right tackle. The Lions were last in the league in rushing and allowed the second-most sacks (63) last season, so improving the offensive line is one of the team's top offseason priorities.
Michael Smith is a senior NFL writer for ESPN.com. ESPN.com senior NFL writer John Clayton contributed to this story.