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Servicing - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Transfers and Servicing [Abstract]
Loans serviced for others unpaid principal amount	$ 275fmb_LoansServicedForOthersUnpaidPrincipalAmount	$ 282fmb_LoansServicedForOthersUnpaidPrincipalAmount
Capitalized service right assets included in other assets	2.0fmb_CapitalizedServiceRightAssetsIncludedInOtherAssets	2.1fmb_CapitalizedServiceRightAssetsIncludedInOtherAssets
Fair market value of the capitalized servicing rights	$ 2.9fmb_CapitalizedServiceRightAssetsFairMarketValue	$ 2.8fmb_CapitalizedServiceRightAssetsFairMarketValue
Prepayment rate	9.60%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed	11.20%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValuePrepaymentSpeed
Discount yield	6.48%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate	8.25%us-gaap_ServicingAssetsAndServicingLiabilitiesAtFairValueAssumptionsUsedToEstimateFairValueDiscountRate

X

- Definition

Capitalized service right assets fair market value.

+ References

No definition available.

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- Definition

Capitalized service right assets included in other assets.

+ References

No definition available.

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- Definition

Loans serviced for others unpaid principal amount.

+ References

No definition available.

+ Details

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X

- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 2

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122596-111746

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X

- Definition

Estimated rate of prepayments of principal on servicing assets and servicing liabilities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 2

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122596-111746

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