This element represents the amount of deposits to be acquired in the transaction.

No definition available.

Name:	fmbi_BusinessCombinationAnnouncedDepositsToBeAcquired
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the amount of loans to be acquired in the transaction.

+ References

No definition available.

+ Details

Name:	fmbi_BusinessCombinationAnnouncedLoansToBeAcquired
Namespace Prefix:	fmbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount represents the number of acquired drive-up locations.

+ References

No definition available.

+ Details

Name:	fmbi_BusinessCombinationNumberofDriveUpLocationsAcquired
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents the number of locations to be acquired.

+ References

No definition available.

+ Details

Name:	fmbi_BusinessCombinationNumberofLocationsToBeAcquired
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount represents the number of acquired branches.

+ References

No definition available.

+ Details

Name:	fmbi_BusinessCombinationNumberofRetailBranchesAcquired
Namespace Prefix:	fmbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6578-128477

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 30

-Section 30

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6911251&loc=d3e6613-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of equity interests of the acquirer, including instruments or interests issued or issuable in consideration for the business combination.

+ References

+ Details

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

- Definition

Amount of assets acquired at the acquisition date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

+ Details

Name:	us-gaap_Goodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fmbi_BancoPopularNorthAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fmbi_GreatLakesFinancialResourcesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fmbi_NationalMachineToolFinancialCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fmbi_PeoplesBancorpInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Acquisitions (Details) $ in Millions	Dec. 02, 2014 USD ($) location	Sep. 26, 2014 USD ($)	Aug. 08, 2014 USD ($) retail_branch	Sep. 21, 2015 USD ($) location
Banco Popular North America
Business Acquisition [Line Items]
Business combination, number of retail branches acquired \| retail_branch			12
Payments to acquire business			$ 19.0
Goodwill			$ 32.2
Great Lakes Financial Resources Inc
Business Acquisition [Line Items]
Business combination, number of retail branches acquired \| location	7
Payments to acquire business	$ 17.5
Goodwill	10.3
Transaction value of acquisition	55.8
Business combination, consideration transferred, equity interests issued	$ 38.3
Business combination, number of drive-up locations acquired \| location	1
National Machine Tool Financial Corporation
Business Acquisition [Line Items]
Payments to acquire business		$ 3.1
Goodwill		4.0
Business combination, recognized identifiable assets acquired and liabilities assumed, assets		$ 5.9
Peoples Bancorp, Inc.
Business Acquisition [Line Items]
Locations to be acquired \| location				2
Announced loans to be acquired				$ 57.0
Announced deposits to be acquired				$ 95.0