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Share-Based Compensation (Detail) - Shares of Company Common Stock Available Under Share-Based Plans
Dec. 31, 2014
Omnibus Stock and Incentive Plan [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares Authorized 8,631,641,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= fmbi_OmnibusStockandIncentivePlanMember
Shares Available For Grant 2,237,337,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= fmbi_OmnibusStockandIncentivePlanMember
Nonemployee Directors Stock Plan  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares Authorized 481,250,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
/ us-gaap_PlanNameAxis
= fmbi_NonemployeeDirectorsStockPlanMember
Shares Available For Grant 75,294,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_PlanNameAxis
= fmbi_NonemployeeDirectorsStockPlanMember