Note 10 - Derivative Financial Instruments (Details) (USD $)	6 Months Ended
Jun. 30, 2013
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Transaction Fees (in Dollars)	$ 522,000
Portion of Fair Value of Outstanding Interest Rate Swaps Covered by Collateral Agreements	100.00%

Transaction fees earned on interest rate derivative contracts. This item is included in other service charges, commission, and fees in Consolidated Statements of Income.

No definition available.

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