Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRS - TDRs That Were Restructured (Details) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017 USD ($) loan	Sep. 30, 2017 USD ($) loan
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	12	14
Pre- Modification Recorded Investment	$ 29,467	$ 30,389
Funds Disbursed	196	196
Interest and Escrow Capitalized	0	0
Charge-offs	1,736	1,736
Post- Modification Recorded Investment	$ 27,927	$ 28,849
Commercial and industrial \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	10	12
Pre- Modification Recorded Investment	$ 25,811	$ 26,733
Funds Disbursed	196	196
Interest and Escrow Capitalized	0	0
Charge-offs	1,736	1,736
Post- Modification Recorded Investment	$ 24,271	$ 25,193
Commercial real estate \| Office, retail, and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	2	2
Pre- Modification Recorded Investment	$ 3,656	$ 3,656
Funds Disbursed	0	0
Interest and Escrow Capitalized	0	0
Charge-offs	0	0
Post- Modification Recorded Investment	$ 3,656	$ 3,656

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

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

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbi_CommercialIndustrialAndAgriculturalMember
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: