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Employee Benefit Plans (Detail) - Pension Plan Asset Allocation (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Asset Category:      
Fair Value of Plan Assets (in Dollars) $ 72,193us-gaap_DefinedBenefitPlanFairValueOfPlanAssets $ 74,370us-gaap_DefinedBenefitPlanFairValueOfPlanAssets $ 63,501us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Equity Securities      
Asset Category:      
Target Plan Asset Allocations, minimum 50.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
   
Target Plan Asset Allocations, maximum 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
   
Fair Value of Plan Assets (in Dollars) 42,826us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
[1]    
Percentage of Plan Assets 59.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
63.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_EquitySecuritiesMember
 
Fixed Income Securities      
Asset Category:      
Target Plan Asset Allocations, minimum 30.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
   
Target Plan Asset Allocations, maximum 48.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
   
Fair Value of Plan Assets (in Dollars) 25,832us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
[1]    
Percentage of Plan Assets 36.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
30.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_FixedIncomeSecuritiesMember
 
Cash Equivalents      
Asset Category:      
Target Plan Asset Allocations, minimum 20.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashEquivalentsMember
   
Target Plan Asset Allocations, maximum 10.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashEquivalentsMember
   
Fair Value of Plan Assets (in Dollars) 3,535us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashEquivalentsMember
[1]    
Percentage of Plan Assets 5.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashEquivalentsMember
7.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= us-gaap_CashEquivalentsMember
 
Total      
Asset Category:      
Fair Value of Plan Assets (in Dollars) $ 72,193us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fmbi_TotalMember
[1]    
Percentage of Plan Assets 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fmbi_TotalMember
100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
/ us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis
= fmbi_TotalMember
 
[1] Additional information regarding the fair value of Pension Plan assets at December 31, 2014 can be found in Note 22, "Fair Value."