Name:	fmbi_AdjustmentToDividendsAvailable
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Item represents percentage that the following categories must exceed in the aggregate to trigger the requirement that all categories be deducted from Common Equity Tier 1 Risk Based Capital: (i) mortgage servicing rights, (ii) deferred tax assets depending on future taxable income, and (iii) significant investments in non-consolidated financial entities.

No definition available.

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

Item represents the capital conversion buffer required in addition to the minimum ratio of Common Equity Tier 1 Capital to Risk Weighted Assets under Basel III capital rules.

No definition available.

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

Capital Required for Capital Adequacy to Risk Weighted Assets, Adjusted

No definition available.

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

Item represents reserve balances maintained at the Federal Reserve Bank. Reserves against deposits are required to be maintained either in the form of vault cash or noninterest-bearing balances maintained with the Federal Reserve Bank and are based on the average daily balances and statutory reserve ratios prescribed by the type of deposit account.

No definition available.

Name:	fmbi_CashReserveBalance
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Item represents the additional percentage of deductions from and adjustments to Common Equity Tier 1 Risk Based Capital that can be phased-in annually.

No definition available.

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

Item represents the percentage of deductions from and adjustments to Common Equity Tier 1 Risk Based Capital that can be phased-in annually.

No definition available.

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

Item represents the period of time the number of deductions from and adjustments to Common Equity Tier 1 Risk Based Capital can be phased-in.

No definition available.

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

Item represents the Common Equity Tier 1 Risk Based Capital to Risk Weighted Assets required under Basel III capital rules.

No definition available.

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

Item represents the minimum common equity tier one risk based capital to risk weighted assets ratio, including the conservation buffer under Basel III capital rules.

No definition available.

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

This item represents the amount of consolidated assets a Bank Holding Company must be under to include trust-preferred securities in Additional Tier 1 capital on a permanent basis and without any phase-out.

No definition available.

Name:	fmbi_MaximumConsolidatedAssetsAllowableToIncludeTrustPreferredSecuritiesInAdditionalTier1Capital
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Item represents percentage that any one of the following categories must exceed to trigger the requirement that the category be deducted from Common Equity Tier 1 Risk Based Capital: (i) mortgage servicing rights, (ii) deferred tax assets depending on future taxable income, and (iii) significant investments in non-consolidated financial entities.

No definition available.

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

Risk weighting under Basel III Capital Rules assigned based on the nature of the assets.

No definition available.

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

Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets, Adjusted

No definition available.

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

This item represents the amount of trust-preferred securities in additional Tier 1 capital.

No definition available.

Name:	fmbi_TierOneRiskBasedCapitalTrustPreferredSecurities
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826

Name:	us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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

The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

Name:	us-gaap_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826