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Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail)
9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
$ / shares
Sep. 30, 2016
USD ($)
security
Investment
Dec. 31, 2016
USD ($)
security
Investment
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities $ 26,994,700,000 $ 23,521,200,000 $ 23,482,600,000
Fixed maturities      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 18,660,000,000 13,916,900,000 16,243,800,000
Fixed maturities | Commercial mortgage-backed securities      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 2,767,900,000 2,278,400,000 2,253,400,000
Equity securities | Nonredeemable preferred stocks      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 813,700,000 860,100,000 853,500,000
Fair Value, Inputs, Level 3      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 5,300,000 1,000,000 700,000
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 300,000 400,000 400,000
Fair Value, Inputs, Level 3 | Subtotal Level 3 Securities      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 5,000,000 $ 600,000 $ 300,000
Fair Value, Inputs, Level 3 | Internal Pricing      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Number of securities | Investment   0 0
Fair Value, Inputs, Level 3 | Fixed maturities      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 0 $ 600,000 $ 300,000
Fair Value, Inputs, Level 3 | Fixed maturities | External Pricing      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 0 $ 600,000 $ 300,000
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | External Pricing      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Number of securities | security   1 1
Available-for-sale Securities 0 $ 600,000 $ 300,000
Valuation Technique   External vendor External vendor
Unobservable Input, prepayment rate   0.00% 0.00%
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities 5,000,000 $ 0 $ 0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Internal Pricing      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Available-for-sale Securities $ 5,000,000 $ 0 $ 0
Valuation Technique Internal price    
Fair Value Inputs, Original Issue Price | $ / shares $ 3.9