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Derivative Instruments and Hedging Activities (Details) - Other Derivative Instruments (Other Derivative Instruments, USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Other Derivative Instruments
    
Derivative [Line Items]    
Gross notional amount outstanding $ 527,893invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember		 $ 256,638invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember
Derivative asset fair value 7,852us-gaap_InterestRateDerivativeAssetsAtFairValue
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember		 2,235us-gaap_InterestRateDerivativeAssetsAtFairValue
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember
Derivative liability fair value (7,852)us-gaap_InterestRateDerivativeLiabilitiesAtFairValue
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember		 (2,235)us-gaap_InterestRateDerivativeLiabilitiesAtFairValue
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember
Fair value of assets needed to settle derivative transactions $ 8,130us-gaap_AssetsNeededForImmediateSettlementAggregateFairValue
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember		 [1] $ 1,305us-gaap_AssetsNeededForImmediateSettlementAggregateFairValue
/ us-gaap_DerivativeByNatureAxis
= fmbi_OtherDerivativeInstrumentsMember		 [1]
[1] This amount represents the fair value if credit risk related contingent factors were triggered.