Commitments, Guarantees, and Contingent Liabilities (Detail) - FHLB Advances - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]
Gain on termination of FHLB forward commitments	$ 0	$ 0	$ 7,829

Amount represents gains from the termination of FHLB forward commitments.

No definition available.

Name:	fmbi_GainOnTerminationOfFHLBForwardCommitments
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration