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Share-Based Compensation (Detail)
12 Months Ended 84 Months Ended 132 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Dec. 31, 2007
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Options, Grants in Period (in shares) 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross      
Options, Exercises in Period (in shares) 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised 0us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised    
Nonemployee Directors Stock Plan | Accelerated Grant Date          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting period       1 year  
Restricted Stock And Restricted Stock Unit Awards          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award vesting period 3 years        
Restricted Stock And Restricted Stock Unit Awards | Vesting After Two Years          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award Vesting Rights, Percentage 50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= fmbi_RestrictedStockAndRestrictedStockUnitAwardsMember
/ us-gaap_VestingAxis
= fmbi_VestingAfterTwoYearsMember
       
Restricted Stock And Restricted Stock Unit Awards | Vesting After Three Years          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award Vesting Rights, Percentage 50.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= fmbi_RestrictedStockAndRestrictedStockUnitAwardsMember
/ us-gaap_VestingAxis
= fmbi_VestingAfterThreeYearsMember
       
Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
ShareBasedCompensationArrangementSharesEarnedBasedOnPerformanceMetrics 3 years        
Share-based compensation arrangement, metric weighting 50.00%fmbi_SharebasedCompensationArrangementMetricWeighting
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
       
Performance Shares | Vesting After Three Years          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award Vesting Rights, Percentage 33.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_VestingAxis
= fmbi_VestingAfterThreeYearsMember
       
Performance Shares | Vesting After Fourth and Fifth Years          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Award Vesting Rights, Percentage 33.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_VestingAxis
= fmbi_VestingAfterFourthandFifthYearsMember
       
Employee Stock Option | Omnibus Stock and Incentive Plan [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Expiration period 10 years        
Employee Stock Option | Nonemployee Directors Stock Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Expiration period         10 years
Minimum | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Shares Earned Based On Three Year Period 0.00%fmbi_SharebasedCompensationArrangementbySharebasedPaymentAwardPercentageofSharesEarnedBasedOnPerformanceMetrics
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
       
Maximum | Performance Shares          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Shares Earned Based On Three Year Period 200.00%fmbi_SharebasedCompensationArrangementbySharebasedPaymentAwardPercentageofSharesEarnedBasedOnPerformanceMetrics
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember