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

No definition available.

Name:	fmbi_NumberOfFormerBranchesHeldForSale
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 after amortization of leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
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+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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Premises, Furniture, and Equipment (Detail) - Narrative $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) location land	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Aug. 08, 2014 USD ($)
Property, Plant and Equipment [Line Items]
Number of former branches held for sale \| location	12
Number of land parcels held for sale \| land	7
Property valuation adjustments	$ 8,581	$ 0	$ 0
Depreciation of premises, furniture, and equipment	13,367	12,224	$ 11,038
Intangible liabilities	$ 9,034
Popular Community Bank
Property, Plant and Equipment [Line Items]
Intangible liabilities		$ 10,600		$ 10,631