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

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

+ References

No definition available.

+ Details

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

- Details

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- Definition

Loans Held-for-Sale, Loan Type

+ References

No definition available.

+ Details

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

- 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 element represents the number of CDOs acquired in the transaction.

+ References

No definition available.

+ Details

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

- Definition

Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Details

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- Details

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- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
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- Details

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Fair Value (Detail) (USD $)	12 Months Ended		3 Months Ended
Fair Value (Detail) (USD $)	Dec. 31, 2014 security	Dec. 31, 2013 security	Dec. 31, 2014 security	Dec. 02, 2014
Fair Value [Line Items]
Number of CDOs Sold	1fmbi_NumberOfCDOsSold	1fmbi_NumberOfCDOsSold
Carrying value of CDO sold	$ 1,300,000fmbi_CarryingValueOfCDOSold	$ 0fmbi_CarryingValueOfCDOSold	$ 1,300,000fmbi_CarryingValueOfCDOSold
1-4 family mortgages
Fair Value [Line Items]
Type of loans held-for-sale	1		1
Commercial Real Estate
Fair Value [Line Items]
Type of loans held-for-sale	1		1
Great Lakes Financial Resources Inc
Fair Value [Line Items]
Securities available-for-sale				219,279,000
Trust-preferred Collateralized Debt Obligations \| Great Lakes Financial Resources Inc
Fair Value [Line Items]
Number of Collateralized Debt Obligations Acquired			4
Securities available-for-sale	$ 2,900,000		$ 2,900,000
Minimum
Fair Value [Line Items]
Appraisal adjustment	0.00%		0.00%
Maximum
Fair Value [Line Items]
Appraisal adjustment	15.00%		15.00%