Subsequent Events (Details) (Great Lakes Financial Resources Inc, Subsequent Event, USD $) In Millions, unless otherwise specified	Jul. 07, 2014
Great Lakes Financial Resources Inc \| Subsequent Event
Subsequent Event [Line Items]
Deposits to be acquired	$ 490
Loans to be acquired	234
Transaction value of announced acquisition	$ 58.0

This amount represents the total estimated transaction value of the announced acquisition, which includes a combination of Company stock and cash.

No definition available.

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