Name:	fmbi_CollateralizedMortgageObligationsAndOtherMortgageBackedSecurities
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:	instant

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_FairValueDiscountedProjectedCashFlowsRateRange1
Namespace Prefix:	fmbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

Portion of other than temporary impairment related to credit losses on debt securities held by the entity at the end of the period recognized in other comprehensive income.

No definition available.

Name:	fmbi_PortionOfOtherThanTemporaryImpairmentRecognizedInAccumulatedOtherComprehensiveLoss
Namespace Prefix:	fmbi_
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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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=7512638&loc=d3e5086-111524