Loans (Details) (USD $)	3 Months Ended
Mar. 31, 2014	Mar. 31, 2013
Loans and Leases Receivable Disclosure [Abstract]
Loans Gains (Losses)	$ 1,100,000	$ 2,000,000
Carrying Amount of Loans Sold	50,800,000	54,000,000
Mortgage Loans Originated With Intent to Sell	$ 15,500,000	$ 0

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