Past Due Loans, Allowances For Credit Losses, Impaired Loans, and TDRs (Detail) - TDRs That Were Restructured (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014 loan	Dec. 31, 2013 loan	Dec. 31, 2012 loan
Financing Receivable, Modifications [Line Items]
Number of Loans	10	39	17
Pre-Modification Recorded Investment	$ 24,619	$ 20,343	$ 7,345
Funds Disbursed	0fmbi_FundsDisbursed	30fmbi_FundsDisbursed	0fmbi_FundsDisbursed
Interest and Escrow Capitalized	0fmbi_InterestAndEscrowCapitalized	63fmbi_InterestAndEscrowCapitalized	4fmbi_InterestAndEscrowCapitalized
Charge-offs	0fmbi_Chargeoffs	0fmbi_Chargeoffs	295fmbi_Chargeoffs
Post-Modification Recorded Investment	24,619	20,436	7,054
Commercial And Industrial
Financing Receivable, Modifications [Line Items]
Number of Loans	7	7	5
Pre-Modification Recorded Investment	23,852	14,439	3,277
Funds Disbursed	0	0	0
Interest and Escrow Capitalized	0	2	0
Charge-offs	0	0	170
Post-Modification Recorded Investment	23,852	14,441	3,107
Office Retail And Industrial
Financing Receivable, Modifications [Line Items]
Number of Loans	1	6	2
Pre-Modification Recorded Investment	417	2,275	2,416
Funds Disbursed	0	30	0
Interest and Escrow Capitalized	0	0	0
Charge-offs	0	0	0
Post-Modification Recorded Investment	417	2,305	2,416
Multi-Family
Financing Receivable, Modifications [Line Items]
Number of Loans	1	5
Pre-Modification Recorded Investment	275	1,274
Funds Disbursed	0	0
Interest and Escrow Capitalized	0	57
Charge-offs	0	0
Post-Modification Recorded Investment	275	1,331
Construction
Financing Receivable, Modifications [Line Items]
Number of Loans		2
Pre-Modification Recorded Investment		508
Funds Disbursed		0
Interest and Escrow Capitalized		0
Charge-offs		0
Post-Modification Recorded Investment		508
Other Commercial Real Estate
Financing Receivable, Modifications [Line Items]
Number of Loans		5	5
Pre-Modification Recorded Investment		526	1,070
Funds Disbursed		0	0
Interest and Escrow Capitalized		0	0
Charge-offs		0	125
Post-Modification Recorded Investment		526	945
Home equity
Financing Receivable, Modifications [Line Items]
Number of Loans	1	13	1
Pre-Modification Recorded Investment	75	1,189	19
Funds Disbursed	0	0	0
Interest and Escrow Capitalized	0	0	0
Charge-offs	0	0	0
Post-Modification Recorded Investment	75	1,189	19
1-4 family mortgages
Financing Receivable, Modifications [Line Items]
Number of Loans		1	4
Pre-Modification Recorded Investment		132	563
Funds Disbursed		0	0
Interest and Escrow Capitalized		4	4
Charge-offs		0	0
Post-Modification Recorded Investment		$ 136	$ 567

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

No definition available.

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- Definition

This item represents the total amount of funds disbrused on troubled debt restructurings related to financing receivables.

+ References

No definition available.

+ Details

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- 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

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- Details

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

+ Details

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

+ References

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- Definition

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

+ References

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