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

This item represents the assumed recovery rate on deferring collateral related to collateralized debt obligations for banks, thrifts, and other depository institutions.

No definition available.

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

This item represents the assumed recovery rate on deferring collateral related to collateralized debt obligations for insurance companies.

No definition available.

Name:	fmbi_DeferringCollateralRecoveryRateForInsuranceCompanies1
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

This item represents the rate range used to derive the projected cash flows on the credit component of the impairment on collateralized debt obligations.

No definition available.

Name:	fmbi_DiscountedProjectedCashFlowsRateRange
Namespace Prefix:	fmbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

This item represents the rate range used to derive the fair value of projected cash flows on the credit component of the impairment on collateralized debt obligations.

No definition available.

Name:	fmbi_FairValueDiscountedProjectedCashFlowsRateRange
Namespace Prefix:	fmbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

The amount of credit losses recognized in earnings on collateralized debt obligations. The amount represents the credit portion of the other-than-temporary impairment on the security, which is calculated as the amount by which the fair value of an investment in collateralized debt obligations is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary.

No definition available.

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

This item represents the assumed prepayment rate for estimating cash flows on collateralized debt obligations for banks with greater than $15 billion in assets that can no longer include hybrid capital instruments, such as trust-preferred securities, in Tier 1 capital beginning January 1, 2013. This prepayment rate is applied in year 3 which is the start of the phase out period for Tier 1 capital treatment.

No definition available.

Name:	fmbi_PrepaymentRateForBanksWithAssetsGreaterThanFifteenBillionYear3
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

This item represents the assumed prepayment rate for estimating cash flows on collateralized debt obligations for banks with greater than $15 billion in assets that can no longer include hybrid capital instruments, such as trust-preferred securities, in Tier 1 capital beginning January 1, 2013. This prepayment rate is applied in years 4 and after which is the start of the phase out period for Tier 1 capital treatment.

No definition available.

Name:	fmbi_PrepaymentRateForBanksWithAssetsGreaterThanFifteenBillionYear4AndAfter
Namespace Prefix:	fmbi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Total of accumulated other comprehensive loss related to that portion of other than temporary impairment (OTTI) due to factors other than credit losses on debt securities categorized either as available-for-sale or held-to-maturity that an entity does not intend to sell and which it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis of the debt security. The amount represented by this item is net of tax, as applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 16C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossNetOfTaxDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The fair value of available-for-sale securities that serve as collateral for borrowings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 01-6

-Paragraph 13

-Subparagraph i

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5086-111524

Name:	us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

No definition available.