This item represents the carrying value of a sold collateralized debt obligation security.

No definition available.

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

- Details

Name:	fmbi_Note3SecuritiesDetailsSecuritiesPortfolioLineItems
Namespace Prefix:	fmbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the number of CDOs sold during the period.

+ References

No definition available.

+ Details

Name:	fmbi_NumberOfCDOsSold
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the total reduction in the cumulative amount of credit losses recognized in earnings for an other than temporary ("OTTI") of collateral debt obligations ("CDO") due to sales of CDO securities during the period.

+ References

No definition available.

+ Details

Name:	fmbi_OtherThanTemporaryImpairmentCreditLossesRecognizedinEarningsReductionsCDOSold
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26805-111562

+ Details

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

- Definition

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

+ References

Reference 1: 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=28368275&loc=d3e5086-111524

+ Details

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

- Definition

The carrying amount of held-to-maturity securities that serve as collateral for borrowings.

+ References

+ Details

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

Securities (Details) (USD $)	3 Months Ended		9 Months Ended
Securities (Details) (USD $)	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 obligation	Sep. 30, 2013 obligation	Dec. 31, 2013
Securities (Details) - Securities Portfolio [Line Items]
Available-for-sale securities pledged as collateral	$ 816,100,000		$ 816,100,000		$ 755,300,000
Held-to-maturity securities pledged as collateral	0		0		0
Available-for-sale securities, gross realized gain (loss), excluding OTTI	2,570,000	34,205,000	8,188,000	34,421,000
Number Of CDOs Sold			1	1
Carrying Value Of CDO Sold	1,300,000	0	1,300,000	0
OTTI credit losses recognized in earnings reductions CDO sold			8,600,000	6,800,000
Corporate Bonds and Other Investments
Securities (Details) - Securities Portfolio [Line Items]
Available-for-sale securities, gross realized gain (loss), excluding OTTI	2,600,000
Trust-preferred collateralized debt obligations
Securities (Details) - Securities Portfolio [Line Items]
Available-for-sale securities, gross realized gain (loss), excluding OTTI			3,500,000
Municipal Securities
Securities (Details) - Securities Portfolio [Line Items]
Available-for-sale securities, gross realized gain (loss), excluding OTTI			468,000
Equity Securities
Securities (Details) - Securities Portfolio [Line Items]
Available-for-sale securities, gross realized gain (loss), excluding OTTI			$ 1,600,000