Note 4 - Loans (Details) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2013
Disclosure Text Block Supplement [Abstract]
Carring Amount of Loans Sold	$ 28,000,000	$ 82,000,000
LoansGainsLosses	$ 1,040,000	$ 3,006,000

Amount represents gains and losses recognized on the sale of portfolio loans and loans held-for-sale subsequent to the date of transfer to held-for-sale.

No definition available.

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