Amount represents the fair value of loans classified as held-for-sale that have a signed sales contract.

No definition available.

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Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount represents the total forward commitment entered into with the FHLB.

+ References

No definition available.

+ Details

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

- Definition

The amount of loans classified as held-for-sale with an agreement to sell at the end of the period to the total amount of loans held-for-sale.

+ References

No definition available.

+ Details

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

- Definition

Premium rate on the repurchase and retirement of junior subordinated debt.

+ References

No definition available.

+ Details

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

- Definition

Premium rate on the repurchase and retirement of subordinated debt.

+ References

No definition available.

+ Details

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

- Definition

Interest rate stated in the contractual debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 26

-Paragraph 20, 21

-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 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 50

-Section 40

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12317-112629

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 50

-Section 40

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6850294&loc=d3e12355-112629

+ Details

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

- Definition

The net cash inflow or outflow associated with security instrument that either represents a creditor or an ownership relationship with the holder of the investment security with a maturity of beyond one year or normal operating cycle, if longer. Includes proceeds from and repayments of (a) debt, (b) capital lease obligations, (c) mandatory redeemable capital securities, and (d) any combination of (a), (b), or (c).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 18

-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_ProceedsFromRepaymentsOfLongTermDebtAndCapitalSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Note 13 - Subsequent Event (Detail) (USD $)	9 Months Ended		3 Months Ended
Note 13 - Subsequent Event (Detail) (USD $)	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2012 Subsequent Event [Member] 6.95% Junior Subordinated Debentures [Member]	Sep. 30, 2012 Subsequent Event [Member] 5.85% Subordinated Debentures [Member]	Sep. 30, 2012 Subsequent Event [Member]
Agreement to Sell Loans Held for Sale (in Dollars)					$ 65,200,000
Percent of Total Loans Held-For-Sale					71.80%
Proceeds from (Repayments of) Long-term Debt and Capital Securities (in Dollars)	(20,004,000)		4,300,000	12,000,000
Debt Instrument, Interest Rate, Stated Percentage		6.95%	6.95%	5.85%
Premium On Retirement Of Junior Subordinated Debt			3.00%
Premium On Retirement Of Subordinated Debt				5.00%
Gains (Losses) on Extinguishment of Debt (in Dollars)					814,000
undefined (in Dollars)	$ 200,000,000				$ 50,000,000