Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRs - TDRs That Were Restructured (Details) $ in Thousands	12 Months Ended
Dec. 31, 2019 USD ($) loan	Dec. 31, 2017 USD ($) loan
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	8	14
Pre-Modification Recorded Investment	$ 18,101	$ 30,389
Funds Disbursed	0	9,035
Interest and Escrow Capitalized	0	0
Charge-offs	2,297	6,232
Post-Modification Recorded Investment	$ 15,804	$ 33,192
Commercial, industrial, and agricultural \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	5	12
Pre-Modification Recorded Investment	$ 13,616	$ 26,733
Funds Disbursed	0	9,035
Interest and Escrow Capitalized	0	0
Charge-offs	1,424	6,232
Post-Modification Recorded Investment	$ 12,192	$ 29,536
Commercial real estate loans \| Office, retail, and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	2	2
Pre-Modification Recorded Investment	$ 4,473	$ 3,656
Funds Disbursed	0	0
Interest and Escrow Capitalized	0	0
Charge-offs	873	0
Post-Modification Recorded Investment	$ 3,600	$ 3,656
Commercial real estate loans \| Multi-family
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	1
Pre-Modification Recorded Investment	$ 12
Funds Disbursed	0
Interest and Escrow Capitalized	0
Charge-offs	0
Post-Modification Recorded Investment	$ 12

This item represents the total amount of charge-offs on troubled debt restructurings related to financing receivables.

No definition available.

Name:	fmbi_Chargeoffs
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents the total amount of funds disbursed on troubled debt restructurings related to financing receivables.

+ References

No definition available.

+ Details

Name:	fmbi_FundsDisbursed
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This item represents the total amount of interest and escrow capitalized on troubled debt restructurings related to financing receivables.

+ References

No definition available.

+ Details

Name:	fmbi_InterestAndEscrowCapitalized
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
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

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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