Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRs (Details) - Narrative - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Modifications [Line Items]
Accruing	$ 2,291,000	$ 2,743,000
Valuation allowance on TDR loans	0	758,000
Commitments to lend additional funds on TDR loans	0	0
Special mention and substandard loans
Financing Receivable, Modifications [Line Items]
Accruing	$ 834,000	$ 862,000

Item represents loans as of the balance sheet date that have been modified from their original contractual terms and are classified as Troubled Debt Restructurings.

No definition available.

+ Details

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

- Definition

Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings.

+ References

No definition available.

+ Details

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

- 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

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

+ Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=fmbi_SpecialMentionandSubstandardLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: