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Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Realized gains and losses on sales	$ 57.9us-gaap_GainsLossesOnSalesOfAssets	$ 91.0us-gaap_GainsLossesOnSalesOfAssets	$ 60.5us-gaap_GainsLossesOnSalesOfAssets
Investments with a carrying value and fair value	$ 176.9us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged	$ 205.2us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged
Redemption period	5 years
Minimum [Member]
Schedule of Available-for-sale Securities [Line Items]
Underwater security	10 years
Loss position existed	3 years

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- Definition

Debt instrument redemption period.

+ References

No definition available.

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- Definition

For under water securities loss position expiration period.

+ References

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- Definition

Maturity period for under water securities.

+ References

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- Definition

Amount of gain (loss) on sale or disposal of assets utilized in financial service operations.

+ References

No definition available.

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- Definition

Sum of the carrying amounts as of the date of the latest financial statement presented of securities which are owned but transferred to another party to serve as collateral to partially or fully secure a debt obligation, repurchase agreement or other current or potential obligation, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

-Subparagraph (b)(1)(i)

-URI http://asc.fasb.org/extlink&oid=34720971&loc=SL6224234-111729

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