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Securities (Detail) (USD $)
 12 Months Ended 3 Months Ended
Dec. 31, 2014
security
Dec. 31, 2013
security
Dec. 31, 2012
Dec. 31, 2014
security
Dec. 02, 2014
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities Pledged as Collateral $ 779,400,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral $ 755,300,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral   $ 779,400,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral  
Held-to-maturity Securities pledged as collateral 0us-gaap_HeldToMaturitySecuritiesPledgedAsCollateral 0us-gaap_HeldToMaturitySecuritiesPledgedAsCollateral   0us-gaap_HeldToMaturitySecuritiesPledgedAsCollateral  
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments 8,125,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments 34,572,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments 2,748,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments    
Cost-method Investments, Realized Gain (Loss)   34,000,000us-gaap_CostMethodInvestmentsRealizedGainLoss      
Number of CDOs Sold 1fmbi_NumberOfCDOsSold 1fmbi_NumberOfCDOsSold      
Carrying value of CDO sold 1,300,000fmbi_CarryingValueOfCDOSold 0fmbi_CarryingValueOfCDOSold   1,300,000fmbi_CarryingValueOfCDOSold  
Other Than Temporary Impairment Credit Losses Recognized in Earnings Reductions CDO Sold 8,600,000fmbi_OtherThanTemporaryImpairmentCreditLossesRecognizedinEarningsReductionsCDOSold        
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold 8,570,000us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold [1] 6,789,000us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold [1] 1,391,000us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold [1]    
Trust-preferred Collateralized Debt Obligations          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments 3,500,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		        
Corporate Bond Securities          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments 2,000,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateBondSecuritiesMember		        
Municipal Securities          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments 468,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmbi_StateAndMunicipalMember		        
Other Investments          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments 2,100,000us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_OtherInvestmentsMember		        
Great Lakes Financial Resources Inc          
Schedule of Available-for-sale Securities [Line Items]          
Securities available-for-sale         219,279,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= fmbi_GreatLakesFinancialResourcesIncMember
Great Lakes Financial Resources Inc | Trust-preferred Collateralized Debt Obligations          
Schedule of Available-for-sale Securities [Line Items]          
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments       0us-gaap_AvailableforsaleSecuritiesGrossRealizedGainLossExcludingOtherThanTemporaryImpairments
/ us-gaap_BusinessAcquisitionAxis
= fmbi_GreatLakesFinancialResourcesIncMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
Number of Collateralized Debt Obligations Acquired       4fmbi_NumberofCollateralizedDebtObligationsAcquired
/ us-gaap_BusinessAcquisitionAxis
= fmbi_GreatLakesFinancialResourcesIncMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
Securities available-for-sale $ 2,900,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= fmbi_GreatLakesFinancialResourcesIncMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		     $ 2,900,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= fmbi_GreatLakesFinancialResourcesIncMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
[1] During the year ended December 31, 2014, one CDO with a carrying value of $1.3 million was sold. In addition, one CDO with a carrying value of zero was sold during the year ended December 31, 2013. These CDOs had OTTI of $8.6 million and $6.8 million, respectively, that were previously recognized in earnings.