Note 22 - Fair Value (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012
Deferring Collateral Recovery Rate For Banks Thrifts And Other Depository Institutions	90.00%
Recovery Rate for Insurance Co	85.00%
Deferring Collateral Recovery Rate For Issuers That Defaulted	100.00%
Seven [Member]
	0
Assets Held-for-sale [Member]
Impairment of Real Estate (in Dollars)	$ 1.3
Minimum [Member] \| Collateral Dependent Impaired Loans [Member]
Appraisal Adjustment	0.00%
Maximum [Member] \| Collateral Dependent Impaired Loans [Member]
Appraisal Adjustment	(20.00%)

Range of potential adjustment to appraised values of collateral-dependent impaired loans.

No definition available.

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

- Definition

No authoritative reference available.

+ References

No definition available.

+ Details

Name:	fmbi_CDONumber
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

This item represents the assumed recovery rate on deferring collateral related to collateralized debt obligations for issuers that have already defaulted.

+ References

No definition available.

+ Details

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

- Definition

The charge against earnings in the period to reduce the carrying amount of real property to fair value.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 144

-Paragraph 26

-Subparagraph b

-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.

+ Details

Name:	us-gaap_ImpairmentOfRealEstate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration