Note 5 - Acquired Loans (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Acquired Loans [Abstract]
Covered Loans Accounted for as Purchased Impaired Loans	$ 25.5	$ 28.1

This element represents covered acquired loans that did not have evidence of impairment on the date of acquisition. However, the Company made an election to account for these loans as purchased impaired loans.

No definition available.

Name:	fmbi_CoveredLoansAccountedForAsPurchasedImpairedLoans
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant