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Fair Value - Additional Information (Detail)
$ in Millions
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value of securities transferred out of Level 2 into Level 1 $ 0.0 $ 0.0
Fair Value $ 27,274.7 $ 23,482.6
Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Vendor Quoted Prices, Percent of FV Hierarchy Level 66.00% 52.00%
Fair Value $ 11,950.4 $ 8,854.0
Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Vendor Quoted Prices, Percent of FV Hierarchy Level 98.00% 99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level 2.00% 1.00%
Fair Value $ 15,319.0 $ 14,627.9
Fair Value, Inputs, Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 5.3 $ 0.7
Internal Pricing | Fair Value, Inputs, Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Number of securities | security 1 0
Fair Value $ 5.0  
Fixed maturities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 20,201.7 $ 16,243.8
Fixed maturities | Fair Value, Inputs, Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 6,645.9 2,894.6
Fixed maturities | Fair Value, Inputs, Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value 13,555.8 13,348.9
Fixed maturities | Fair Value, Inputs, Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair Value $ 0.0 $ 0.3