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Goodwill and Other Intangible Assets (Detail) - Core Deposit Intangibles (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Finite-lived Intangible Assets [Roll Forward]      
Balance at the beginning of the year, gross $ 33,775us-gaap_FiniteLivedCoreDepositsGross $ 33,775us-gaap_FiniteLivedCoreDepositsGross $ 34,318us-gaap_FiniteLivedCoreDepositsGross
Balance at the beginning of the year, accumulated amortization 21,471us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 18,193us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 16,145us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Balance at the beginning of the year, net 12,304us-gaap_FiniteLivedIntangibleAssetsNet 15,582us-gaap_FiniteLivedIntangibleAssetsNet 18,173us-gaap_FiniteLivedIntangibleAssetsNet
Additions 14,195us-gaap_FinitelivedIntangibleAssetsAcquired1 0us-gaap_FinitelivedIntangibleAssetsAcquired1 781us-gaap_FinitelivedIntangibleAssetsAcquired1
Amortization expense (2,889)us-gaap_AmortizationOfIntangibleAssets (3,278)us-gaap_AmortizationOfIntangibleAssets (3,372)us-gaap_AmortizationOfIntangibleAssets
Fully amortized assets, gross 0fmbi_FullyAmortizedIntangibleAssetsother 0fmbi_FullyAmortizedIntangibleAssetsother (1,324)fmbi_FullyAmortizedIntangibleAssetsother
Fully amortized assets, net 0fmbi_FullyAmortizedAssetsOtherRemoved 0fmbi_FullyAmortizedAssetsOtherRemoved (1,324)fmbi_FullyAmortizedAssetsOtherRemoved
Balance at the end of the year, gross 47,970us-gaap_FiniteLivedCoreDepositsGross 33,775us-gaap_FiniteLivedCoreDepositsGross 33,775us-gaap_FiniteLivedCoreDepositsGross
Balance at the end of the year, accumulated amortization 24,360us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 21,471us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization 18,193us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Balance at the end of the year, net $ 23,610us-gaap_FiniteLivedIntangibleAssetsNet $ 12,304us-gaap_FiniteLivedIntangibleAssetsNet $ 15,582us-gaap_FiniteLivedIntangibleAssetsNet
Weighted-average remaining life (in years) 8 years 5 years 10 months 24 days 6 years 4 months 24 days
Minimum      
Finite-lived Intangible Assets [Roll Forward]      
Estimated useful lives (in years) 3 months 18 days 2 months 12 days 8 months 12 days
Maximum      
Finite-lived Intangible Assets [Roll Forward]      
Estimated useful lives (in years) 10 years 3 months 18 days 11 years 3 months 18 days 12 years 3 months 18 days