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Securities (Detail) - Securities Gains (Losses) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Gains (losses) on sales of securities:      
Gross realized gains $ 8,188,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains $ 34,572,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains $ 3,045,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross realized losses (63,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses 0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses (297,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Net realized gains on sales of securities 8,125,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments 34,572,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments 2,748,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
Non-cash impairment charges:      
OTTI (28,000)us-gaap_ImpairmentOfInvestments (408,000)us-gaap_ImpairmentOfInvestments (3,728,000)us-gaap_ImpairmentOfInvestments
Portion of OTTI recognized in other comprehensive income (loss) 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest 59,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest
Net non-cash impairment charges (28,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet (408,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet (3,669,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
Net realized gains (losses) 8,097,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet 34,164,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet (921,000)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
Net trading gains (losses) 677,000us-gaap_TradingGainsLosses [1] 3,189,000us-gaap_TradingGainsLosses [1] 1,627,000us-gaap_TradingGainsLosses [1]
Net non-cash impairment charges:      
Net non-cash impairment charges 28,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet 408,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet 3,669,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
Other Mortgage-Backed Securities      
Non-cash impairment charges:      
Net non-cash impairment charges (28,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember		 (6,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember		 (1,443,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember
Net non-cash impairment charges:      
Net non-cash impairment charges 28,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember		 6,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember		 1,443,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedMortgageObligationsMember
Municipal Securities      
Non-cash impairment charges:      
Net non-cash impairment charges 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_StateAndMunicipalMember		 (402,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_StateAndMunicipalMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_StateAndMunicipalMember
Net non-cash impairment charges:      
Net non-cash impairment charges 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_StateAndMunicipalMember		 402,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_StateAndMunicipalMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_StateAndMunicipalMember
Trust-preferred Collateralized Debt Obligations      
Non-cash impairment charges:      
Net non-cash impairment charges 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedDebtObligationsMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedDebtObligationsMember		 (2,226,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedDebtObligationsMember
Net non-cash impairment charges:      
Net non-cash impairment charges 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedDebtObligationsMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedDebtObligationsMember		 2,226,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= us-gaap_CollateralizedDebtObligationsMember
Total      
Non-cash impairment charges:      
Net non-cash impairment charges (28,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_TotalMember		 (408,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_TotalMember		 (3,669,000)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_TotalMember
Net non-cash impairment charges:      
Net non-cash impairment charges $ 28,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_TotalMember		 $ 408,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_TotalMember		 $ 3,669,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_DebtSecurityAxis
= fmbi_TotalMember
[1] All net trading gains relate to trading securities still held as of December 31, 2014, 2013, and 2012 and are included in other income in the Consolidated Statements of Income.