Note 18 - Regulatory and Capital Matters (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cash Reserve Balance	$ 50.9	$ 18.3
Adjustment To Dividends Available	$ 2.2

Item represents reserve balances maintained at the Federal Reserve Bank. Reserves against deposits are required to be maintained either in the form of vault cash or noninterest-bearing balances maintained with the Federal Reserve Bank and are based on the average daily balances and statutory reserve ratios prescribed by the type of deposit account.

No definition available.

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