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Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Debt Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost $ 10,106 $ 9,799
Gross Unrealized Gains 431 359
Gross Unrealized Losses 35 73
Fair Value 10,502 10,085
Debt Securities | Corporate    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 5,530 5,555
Gross Unrealized Gains 281 252
Gross Unrealized Losses 14 26
Fair Value 5,797 5,781
Debt Securities | States, municipalities and political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 4,029 3,770
Gross Unrealized Gains 142 100
Gross Unrealized Losses 17 42
Fair Value 4,154 3,828
Debt Securities | Commercial mortgage-backed    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 282 282
Gross Unrealized Gains 8 7
Gross Unrealized Losses 1 2
Fair Value 289 287
Debt Securities | Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 235 167
Gross Unrealized Gains 0 0
Gross Unrealized Losses 3 3
Fair Value 232 164
Debt Securities | United States government    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 15 10
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 15 10
Debt Securities | Foreign government    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 10 10
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 10 10
Debt Securities | Convertibles and bonds with warrants attached    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 5 5
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 5 5
Equity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 3,218 2,995
Gross Unrealized Gains 2,597 2,348
Gross Unrealized Losses 16 9
Fair Value 5,799 5,334
Equity Securities | Common equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 3,038 2,812
Gross Unrealized Gains 2,559 2,320
Gross Unrealized Losses 16 9
Fair Value 5,581 5,123
Equity Securities | Nonredeemable preferred equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 180 183
Gross Unrealized Gains 38 28
Gross Unrealized Losses 0 0
Fair Value 218 211
Fixed Maturities And Equity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 13,324 12,794
Gross Unrealized Gains 3,028 2,707
Gross Unrealized Losses 51 82
Fair Value $ 16,301 $ 15,419