Note 6 - Past Due Loans, Allowance For Credit Losses, and Impaired Loans (Detail) (USD $)	Dec. 31, 2011
Valuation Allowance On Troubled Debt Restructured Loans	$ 94,000

Item represents the amount of allowance for loan losses related to troubled debt restructured loans as of the balance sheet date.

No definition available.

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