Summary of Significant Accounting Policies (Details)	12 Months Ended
Dec. 31, 2018 segment subsidiary
Property, Plant and Equipment [Line Items]
Number of wholly-owned subsidiaries \| subsidiary	3
Number of reportable segments \| segment	1
Finite-lived intangible assets amortization period	13 years
Furniture And Equipment \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	3 years
Furniture And Equipment \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	10 years
Premises \| Minimum
Property, Plant and Equipment [Line Items]
Useful life	25 years
Premises \| Maximum
Property, Plant and Equipment [Line Items]
Useful life	40 years

This item represents the number of wholly owned subsidiaries by the Company.

No definition available.

Name:	fmbi_NumberOfWhollyOwnedSubsidiaries
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Remaining amortization period of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsRemainingAmortizationPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
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_PropertyPlantAndEquipmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
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- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_LandBuildingsAndImprovementsMember
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Period Type: