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Fair Value Fair Value - Additional Information (Detail)
Sep. 30, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Sep. 30, 2017
USD ($)
security
Fair Value Measurements [Line Items]      
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount $ 0 $ 0 $ 0
Investments, Fair Value $ 32,350,600,000 $ 27,274,700,000 $ 26,994,700,000
Fair Value, Inputs, Level 1      
Fair Value Measurements [Line Items]      
Vendor Quoted Prices, Percent of FV Hierarchy Level 76.00% 66.00% 59.00%
Investments, Fair Value $ 15,469,300,000 $ 11,950,400,000 $ 11,086,000,000
Fair Value, Inputs, Level 2      
Fair Value Measurements [Line Items]      
Vendor Quoted Prices, Percent of FV Hierarchy Level 99.00% 98.00% 98.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level 1.00% 2.00% 2.00%
Investments, Fair Value $ 16,876,000,000 $ 15,319,000,000 $ 15,903,400,000
Fair Value, Inputs, Level 3      
Fair Value Measurements [Line Items]      
Investments, Fair Value 5,300,000 5,300,000 5,300,000
Available-for-sale Securities      
Fair Value Measurements [Line Items]      
Investments, Fair Value 28,452,400,000    
Available-for-sale Securities | Fair Value, Inputs, Level 1      
Fair Value Measurements [Line Items]      
Investments, Fair Value 12,336,000,000    
Available-for-sale Securities | Fair Value, Inputs, Level 2      
Fair Value Measurements [Line Items]      
Investments, Fair Value 16,116,400,000    
Available-for-sale Securities | Fair Value, Inputs, Level 3      
Fair Value Measurements [Line Items]      
Investments, Fair Value 0    
Nonredeemable preferred stocks | Internal Pricing | Fair Value, Inputs, Level 3      
Fair Value Measurements [Line Items]      
Investments, Fair Value $ 5,000,000 $ 5,000,000 $ 5,000,000
Number of securities | security 1 1 1