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Investments - Financial Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value $ 4,064.4us-gaap_InvestmentsFairValueDisclosure $ 4,097.9us-gaap_InvestmentsFairValueDisclosure
Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 3,648.4us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueMeasurementsRecurringMember		 3,700.1us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueMeasurementsRecurringMember
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 855.7us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueMeasurementsRecurringMember		 [1]
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 2,791.9us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueInputsLevel2Member
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= us-gaap_FairValueMeasurementsRecurringMember		 [2] 2,840.7us-gaap_InvestmentsFairValueDisclosure
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Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0.8us-gaap_InvestmentsFairValueDisclosure
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Other Investments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 520.9us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_OtherInvestmentsMember		 495.1us-gaap_InvestmentsFairValueDisclosure
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Other Investments [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 104.9us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueMeasurementsRecurringMember		 97.3us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueMeasurementsRecurringMember
Other Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueMeasurementsRecurringMember		 [1]
Other Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 104.9us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FairValueInputsLevel2Member
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= us-gaap_FairValueMeasurementsRecurringMember		 [2]
Other Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 2,788.6us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 2,840.7us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 182.8us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Non-U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 81.5us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 480.0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Credit-Financial [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 484.7us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Credit-Industrial [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 469.9us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Credit-Utility [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 147.2us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | CMO/MBS-Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 168.7us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | CMO/MBS-Non Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 13.4us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | CMBS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 186.0us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_DebtSecurityAxis
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= us-gaap_FixedMaturitiesMember		 [5] 180.0us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_CommercialMortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 [5]
Fixed Maturities [Member] | ABS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 223.4us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FixedMaturitiesMember		 [6]
Fixed Maturities [Member] | Foreign Denominated: Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 141.6us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_DebtSecurityAxis
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= us-gaap_FixedMaturitiesMember		 140.2us-gaap_InvestmentsFairValueDisclosure
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= us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Foreign Denominated: Credit [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 209.4us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_DebtSecurityAxis
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 2,788.6us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 182.8us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Non-U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 81.5us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 480.0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Financial [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 484.7us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Industrial [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 469.9us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Utility [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 147.2us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 168.7us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Non Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 13.4us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 186.0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | ABS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 223.4us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 141.6us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Credit [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 209.4us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 102.0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 102.0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 [3]
Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Non-U.S. Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Financial [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Industrial [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Credit-Utility [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | CMO/MBS-Non Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | CMBS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | ABS [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Governments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fixed Maturities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member] | Foreign Denominated: Credit [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 483.4us-gaap_InvestmentsFairValueDisclosure
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Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
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Equity Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 482.6us-gaap_InvestmentsFairValueDisclosure
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Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0us-gaap_InvestmentsFairValueDisclosure
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Equity Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0.8us-gaap_InvestmentsFairValueDisclosure
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Short-Term Investments [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 271.5us-gaap_InvestmentsFairValueDisclosure
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Short-Term Investments [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 271.5us-gaap_InvestmentsFairValueDisclosure
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Short-Term Investments [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 271.1us-gaap_InvestmentsFairValueDisclosure
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Short-Term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fair Value 0.4us-gaap_InvestmentsFairValueDisclosure
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Short-Term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
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[1] Quoted prices in active markets for identical assets
[2] Significant other observable inputs
[3] Significant unobservable inputs
[4] Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).
[5] Commercial mortgage-backed securities (“CMBS”).
[6] Asset-backed securities (“ABS”).