Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRs - TDRs That Were Restructured (Details) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	14	2
Pre-Modification Recorded Investment	$ 30,389	$ 6,205
Funds Disbursed	9,035	0
Interest and Escrow Capitalized	0	0
Charge-offs	6,232	1,083
Post-Modification Recorded Investment	$ 33,192	$ 5,122
Commercial, industrial, and agricultural \| Commercial and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	12
Pre-Modification Recorded Investment	$ 26,733
Funds Disbursed	9,035
Interest and Escrow Capitalized	0
Charge-offs	6,232
Post-Modification Recorded Investment	$ 29,536
Commercial real estate loans \| Office, retail, and industrial
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan	2	1
Pre-Modification Recorded Investment	$ 3,656	$ 5,460
Funds Disbursed	0	0
Interest and Escrow Capitalized	0	0
Charge-offs	0	1,083
Post-Modification Recorded Investment	$ 3,656	$ 4,377
Commercial real estate loans \| Other commercial real estate
Financing Receivable, Modifications [Line Items]
Number of Loans \| loan		1
Pre-Modification Recorded Investment		$ 745
Funds Disbursed		0
Interest and Escrow Capitalized		0
Charge-offs		0
Post-Modification Recorded Investment		$ 745

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

No definition available.

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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

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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

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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_
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Namespace Prefix:
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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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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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Namespace Prefix:
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