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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 | $ 57.9 us-gaap_GainsLossesOnSalesOfAssets | $ 91.0 us-gaap_GainsLossesOnSalesOfAssets | $ 60.5 us-gaap_GainsLossesOnSalesOfAssets
Investments with a carrying value and fair value | $ 176.9 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesPledged | $ 205.2 us-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. + Details
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- Definition For under water securities loss position expiration period. + References No definition available. + Details
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- Definition Maturity period for under water securities. + References No definition available. + Details
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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 + Details
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- Details
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