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

No definition available.

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

[Line Items] for Fair Value [Table]

+ References

No definition available.

+ 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 amount represents the number of former branches held for sale.

+ References

No definition available.

+ Details

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

- Definition

This amount represents the number of land parcels previously purchased for expansion held for sale.

+ References

No definition available.

+ Details

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

- 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=56956791&loc=d3e4845-128472

+ Details

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- Details

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Namespace Prefix:
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- Details

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- Details

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Fair Value (Detail) - Narrative	12 Months Ended
Fair Value (Detail) - Narrative	Dec. 31, 2014 USD ($) security loan	Dec. 31, 2013 USD ($) security	Dec. 31, 2015 location land loan	Dec. 02, 2014 USD ($)
Fair Value [Line Items]
Number of CDOs sold \| security	1	1
Carrying value of CDO sold	$ 1,300,000	$ 0
Number of former branches held for sale \| location			12
Number of land parcels held for sale \| land			7
Commercial Real Estate
Fair Value [Line Items]
Type of loans held-for-sale \| loan	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 CDOs sold \| security	4
Securities available-for-sale	$ 2,900,000
Minimum
Fair Value [Line Items]
Appraisal adjustment			0.00%
Maximum
Fair Value [Line Items]
Appraisal adjustment			15.00%