Loans - Loans Sales (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total net gains on loan sales	$ 6,467	$ 4,601	$ 16,444	$ 7,230
Corporate Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	128,453	36,152	303,678	182,613
Corporate Loans \| Multi- family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	11,880	0	26,129	0
Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	371,437	179,410	839,294	394,552
Corporate loan sales
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales	291	295	17,227	4,598
Less book value of loans sold	286	289	17,177	4,477
Total net gains on loan sales	5	6	50	121
Commercial and industrial \| Corporate Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	80,679	22,894	234,075	168,716
Office, retail, and industrial \| Corporate Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	0	0	7,438	0
Construction \| Corporate Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	894	3,258	1,036	3,897
Other commercial real estate \| Corporate Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	35,000	10,000	35,000	10,000
Home equity \| Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	0	0	0	144,967
1-4 family mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Proceeds from sales	214,246	173,251	508,076	292,410
Less book value of loans sold	207,784	168,656	491,682	285,301
Total net gains on loan sales	6,462	4,595	16,394	7,109
1-4 family mortgages \| Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	224,819	179,410	585,918	249,585
Installment \| Consumer Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for loan purchases	$ 146,618	$ 0	$ 253,376	$ 0

The carrying amount of loans sold before adjustment for write-downs on loans held-for-sale.

No definition available.

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

- Definition

Amount of increase from purchase of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919244-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

Amount of decrease from sale and reclassification to held-for-sale of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 11
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919244-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableSignificantSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) on sale of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121570589&loc=d3e107425-111719

+ Details

Name:	us-gaap_GainLossOnSaleOfNotesReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fmbi_TotalCorporateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fmbi_CommercialAndIndustrialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fmbi_OfficeRetailAndIndustrialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fmbi_OtherCommercialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_HomeEquityLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fmbi_InstallmentLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: