Name:	fmbi_ActuarialLossesAsAPercentageOfAccumulatedBenefitObligation
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The maximum contribution a qualified employee can make into the defined contribution retirement savings plan as a percentage of pre-tax base salary.

No definition available.

Name:	fmbi_DefinedContributionPlanEmployeeContributionLimitPercentageOfCompensation
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

This item represents the employer's automatic contribution into the defined contribution retirement savings plan as a percentage of employee eligible compensation.

No definition available.

Name:	fmbi_DefinedContributionPlanEmployerAutomaticContributionLimitPercent
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the vesting period for the Company's automatic and discretionary contribution in the defined contribution retirement savings plan (Profit Sharing Plan).

No definition available.

Name:	fmbi_DefinedContributionPlanEmployerMatchingContributionPercentofEmployeesGrossPayDiscretionaryComponentVestingPeriod
Namespace Prefix:	fmbi_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

This item represents the employer's transition contribution into the defined contribution retirement savings plan for certain employees who were active participants in the defined benefit retirement plan (the Pension Plan) as a percentage of employee eligible compensation.

No definition available.

Name:	fmbi_DefinedContributionPlanEmployerTransitionContribution
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis=fmbi_ContributionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis=fmbi_CertainHighlyCompensatedEmployeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_DeferredCompensationArrangementWithIndividualExcludingShareBasedPaymentsAndPostretirementBenefitsByTypeOfDeferredCompensationAxis=fmbi_Voluntary401KPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: