Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRs - TDRs That Were Restructured (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan	Dec. 31, 2018 USD ($)
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	2	8
Pre-Modification Recorded Investment	$ 10,428	$ 18,101
Funds Disbursed	(3,818)	0
Interest and Escrow Capitalized	0	0
Charge-offs	0	2,297
Post-Modification Recorded Investment	6,610	15,804
Accruing
Financing Receivable, Modifications [Line Items]
Post-Modification Recorded Investment	0	12	$ 0
Non-accrual
Financing Receivable, Modifications [Line Items]
Post-Modification Recorded Investment	$ 10,428	$ 18,089	$ 527
Commercial, industrial, and agricultural \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	2	5
Pre-Modification Recorded Investment	$ 10,428	$ 13,616
Funds Disbursed	(3,818)	0
Interest and Escrow Capitalized	0	0
Charge-offs	0	1,424
Post-Modification Recorded Investment	$ 6,610	$ 12,192
Commercial real estate loans \| Office, retail, and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan		2
Pre-Modification Recorded Investment		$ 4,473
Funds Disbursed		0
Interest and Escrow Capitalized		0
Charge-offs		873
Post-Modification Recorded Investment		$ 3,600
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=121611835&loc=SL6953676-111524

+ Details

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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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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
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Namespace Prefix:
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- Details

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