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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
Mar. 31, 2017
Dec. 31, 2016
Debt Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost $ 9,981 $ 9,799
Gross Unrealized Gains 378 359
Gross Unrealized Losses 58 73
Fair Value 10,301 10,085
Debt Securities | Corporate    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 5,570 5,555
Gross Unrealized Gains 262 252
Gross Unrealized Losses 17 26
Fair Value 5,815 5,781
Debt Securities | States, municipalities and political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 3,879 3,770
Gross Unrealized Gains 110 100
Gross Unrealized Losses 37 42
Fair Value 3,952 3,828
Debt Securities | Commercial mortgage-backed    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 282 282
Gross Unrealized Gains 6 7
Gross Unrealized Losses 1 2
Fair Value 287 287
Debt Securities | Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 219 167
Gross Unrealized Gains 0 0
Gross Unrealized Losses 3 3
Fair Value 216 164
Debt Securities | United States government    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 16 10
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 16 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 3,240 2,995
Gross Unrealized Gains 2,445 2,348
Gross Unrealized Losses 9 9
Fair Value 5,676 5,334
Equity Securities | Common equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 3,051 2,812
Gross Unrealized Gains 2,411 2,320
Gross Unrealized Losses 9 9
Fair Value 5,453 5,123
Equity Securities | Nonredeemable preferred equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 189 183
Gross Unrealized Gains 34 28
Gross Unrealized Losses 0 0
Fair Value 223 211
Fixed Maturities And Equity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 13,221 12,794
Gross Unrealized Gains 2,823 2,707
Gross Unrealized Losses 67 82
Fair Value $ 15,977 $ 15,419