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Past Due Loans, Allowances For Credit Losses, Impaired Loans, and TDRs (Detail) - TDRs That Defaulted Within Twelve Months of the Restructured Date (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
loan
Dec. 31, 2013
loan
Dec. 31, 2012
loan
Number of loans 3us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 4us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 5us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Recorded investment (in Dollars) $ 202us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 $ 704us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 $ 1,616us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Commercial And Industrial      
Number of loans 2us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_CommercialAndIndustrialMember		 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_CommercialAndIndustrialMember		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_CommercialAndIndustrialMember
Recorded investment (in Dollars) 125us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_CommercialAndIndustrialMember		 350us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_CommercialAndIndustrialMember		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_CommercialAndIndustrialMember
Office Retail And Industrial      
Number of loans 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OfficeRetailAndIndustrialMember		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OfficeRetailAndIndustrialMember		 2us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OfficeRetailAndIndustrialMember
Recorded investment (in Dollars) 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OfficeRetailAndIndustrialMember		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OfficeRetailAndIndustrialMember		 837us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OfficeRetailAndIndustrialMember
Other Commercial Real Estate      
Number of loans 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OtherCommercialRealEstateMember		 3us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OtherCommercialRealEstateMember		 2us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OtherCommercialRealEstateMember
Recorded investment (in Dollars) 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OtherCommercialRealEstateMember		 354us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OtherCommercialRealEstateMember		 717us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_OtherCommercialRealEstateMember
Home Equity 1      
Number of loans 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_HomeEquity1Member		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_HomeEquity1Member		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_HomeEquity1Member
Recorded investment (in Dollars) 77us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_HomeEquity1Member		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_HomeEquity1Member		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fmbi_HomeEquity1Member
1-4 family mortgages      
Number of loans 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember		 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember		 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember
Recorded investment (in Dollars) $ 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember		 $ 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember		 $ 62us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ResidentialMortgageMember