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Transfers of Assets between Level 1 and Level 2 (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Transfers Between Level 1 and Level 2 |  |
Level 1 from Level 2 Transfers | $ 1,837.1 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount | $ 1,607.1 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount
Level 2 from Level 1 Transfers | 1,851.7 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount | 1,191.7 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount
United States Government and Government Agencies and Authorities |  |
Transfers Between Level 1 and Level 2 |  |
Level 1 from Level 2 Transfers | 163.2 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 62.2 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
Level 2 from Level 1 Transfers | 0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember | 0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasuryAndGovernmentMember
States, Municipalities, and Political Subdivisions |  |
Transfers Between Level 1 and Level 2 |  |
Level 1 from Level 2 Transfers | 0 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Level 2 from Level 1 Transfers | 0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 53.0 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Public Utilities |  |
Transfers Between Level 1 and Level 2 |  |
Level 1 from Level 2 Transfers | 81.8 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PublicUtilityBondsMember | 248.4 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PublicUtilityBondsMember
Level 2 from Level 1 Transfers | 253.4 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PublicUtilityBondsMember | 20.8 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PublicUtilityBondsMember
Corporate Debt Securities |  |
Transfers Between Level 1 and Level 2 |  |
Level 1 from Level 2 Transfers | 1,592.1 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 1,296.5 us-gaap_FairValueAssetsLevel2ToLevel1TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Level 2 from Level 1 Transfers | $ 1,598.3 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | $ 1,117.9 us-gaap_FairValueAssetsLevel1ToLevel2TransfersAmount / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember

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- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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