Past Due Loans, Allowance For Credit Losses, Impaired Loans and TDRS (Details) - Narrative - USD ($)	Jun. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable, Gross, Allowance, and Net [Abstract]
Valuation allowance on TDR loans	$ 0	$ 758,000
Commitments to lend additional funds TDRs	$ 0	$ 0

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