Acquired Loans and Covered Loans (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Acquired Loans [Line Items]
Purchased impaired loans	$ 83,394,000	$ 119,133,000
Non-purchased impaired loans	739,589,000fmbi_OtherLoans	47,854,000fmbi_OtherLoans
Total	822,983,000fmbi_TotalAcquiredLoans	166,987,000fmbi_TotalAcquiredLoans
Non Covered Loan
Acquired Loans [Line Items]
Purchased impaired loans	28,712,000	15,608,000
Non-purchased impaired loans	714,836,000	17,024,000
Total	743,548,000	32,632,000
Covered Loan
Acquired Loans [Line Items]
Purchased impaired loans	54,682,000	103,525,000
Non-purchased impaired loans	24,753,000	30,830,000
Total	79,435,000	134,355,000
January 1, 2015
Acquired Loans [Line Items]
Number of FDIC assisted transactions no longer covered	1
Financing receivables and OREO, cumulative losses, anticipated FDIC coverage loss	$ 10,100,000
July 1, 2015 and October 1, 2015
Acquired Loans [Line Items]
Number of FDIC assisted transactions no longer covered	2

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

This element reflects the carrying value of certain loans and OREO acquired in Federal Deposit Insurance Corporation (“FDIC”) assisted transactions that will no longer be covered by loss share agreements with the FDIC after a specified date.

+ References

No definition available.

+ Details

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

- Definition

This element reflects the number of Federal Deposit Insurance Corporation (“FDIC”) assisted transactions that will no longer be covered by loss share agreements with the FDIC after a specified date.

+ References

No definition available.

+ Details

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

- Definition

This element represents covered and non-covered loans that did not have evidence of impairment on the date of acquisition and covered and non-covered home equity lines of credit which are open-end consumer loans that are not categorized as purchased impaired loans.

+ References

No definition available.

+ Details

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

- Definition

Aggregate carrying value of all acquired covered and non-covered loans as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

The balance of financing receivables that were acquired with deteriorated credit quality.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (h)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Details

Name:	fmbi_EffectivePeriodDateAxis=fmbi_JanuaryFirst2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fmbi_EffectivePeriodDateAxis=fmbi_JulyFirst2015andOctoberFirst2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbi_CoveredLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbi_NonCoveredLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: