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Fair Value Fair Value - Additional Information (Detail)
Jun. 30, 2018
USD ($)
security
Dec. 31, 2017
USD ($)
security
Jun. 30, 2017
USD ($)
Fair Value Measurements [Line Items]      
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount $ 0 $ 0 $ 0
Fair Value $ 30,921,200,000 $ 27,274,700,000 $ 25,978,400,000
Fair Value, Inputs, Level 1      
Fair Value Measurements [Line Items]      
Vendor Quoted Prices, Percent of FV Hierarchy Level 72.00% 66.00% 58.00%
Fair Value $ 14,049,000,000 $ 11,950,400,000 $ 9,788,000,000
Fair Value, Inputs, Level 2      
Fair Value Measurements [Line Items]      
Vendor Quoted Prices, Percent of FV Hierarchy Level 98.00% 98.00% 99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level 2.00% 2.00% 1.00%
Fair Value $ 16,866,900,000 $ 15,319,000,000 $ 16,190,100,000
Fair Value, Inputs, Level 3      
Fair Value Measurements [Line Items]      
Fair Value 5,300,000 5,300,000 $ 300,000
Available-for-sale Securities      
Fair Value Measurements [Line Items]      
Fair Value 27,020,400,000    
Available-for-sale Securities | Fair Value, Inputs, Level 1      
Fair Value Measurements [Line Items]      
Fair Value 10,829,700,000    
Available-for-sale Securities | Fair Value, Inputs, Level 2      
Fair Value Measurements [Line Items]      
Fair Value 16,190,700,000    
Available-for-sale Securities | Fair Value, Inputs, Level 3      
Fair Value Measurements [Line Items]      
Fair Value 0    
Nonredeemable preferred stocks | Internal Pricing | Fair Value, Inputs, Level 3      
Fair Value Measurements [Line Items]      
Fair Value $ 5,000,000 $ 5,000,000  
Number of securities 1 1 0