Acquired and Covered Loans Acquired and Covered Loans (Details) - Narrative $ in Millions	Dec. 31, 2015 USD ($) transaction
Acquired Loans [Abstract]
Number of FDIC assisted transactions no longer covered \| transaction	3
Financing receivables and other real estate owned FDIC coverage loss	$ 21.0
Renewed non-purchased credit impaired loans	$ 61.6

This element reflects the number of Federal Deposit Insurance Corporation (“FDIC”) assisted transactions with Loss Share Agreements relating to non-residential mortgage loans and OREO that expired and will no longer be covered.

No definition available.

Name:	fmbi_NumberOfFDICAssistedTransactionsWithExpiredLossShareAgreementsRelatingToNonresidentialMortgageLoansAndOREO
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant