Note 9 - Derivative Financial Instruments (Detail) - Interest Rate Derivatives Portfolio (Fair Value Hedges [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Fair Value Hedges [Member]
Related to fixed rate commercial loans
Notional amount outstanding	$ 15,581	$ 15,860
Derivative liability fair value	(2,090)	(2,270)
Weighted-average interest rate received	2.12%	2.12%
Weighted-average interest rate paid	6.39%	6.39%
Weighted-average maturity (in years)	4.51	4.76
Cash pledged to collateralize net unrealized losses with counterparties (1)	2,154	2,516
Fair value of assets needed to settle derivative transactions (2)	$ 2,120	$ 2,301

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

This element represents the weighted average interest rate paid on fair value hedges.

+ References

No definition available.

+ Details

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

- Definition

This element represents the weighted average interest rate received on fair value hedges.

+ References

No definition available.

+ Details

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

- Definition

This element represents the weighted average maturity in years on fair value hedges.

+ References

No definition available.

+ Details

Name:	fmbi_WeightedaverageMaturityinYears
Namespace Prefix:	fmbi_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

+ Details

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

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph c

-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 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

+ Details

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

- Definition

Fair value as of the balance sheet date of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 10

-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_InterestRateDerivativeLiabilitiesAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate notional amount of all derivatives designated as a fair value hedging instrument. The notional amount relates to a number of currency units, shares, bushels, pounds, or other units specified in a derivative instrument.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104

+ Details

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