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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, 2016
Dec. 31, 2015
Debt Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost $ 9,442 $ 9,324
Gross Unrealized Gains 510 431
Gross Unrealized Losses 68 105
Fair Value 9,884 9,650
Debt Securities | Corporate    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 5,395 5,294
Gross Unrealized Gains 295 255
Gross Unrealized Losses 66 96
Fair Value 5,624 5,453
Debt Securities | States, municipalities and political subdivisions    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 3,499 3,440
Gross Unrealized Gains 204 172
Gross Unrealized Losses 0 1
Fair Value 3,703 3,611
Debt Securities | Commercial mortgage-backed    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 289 287
Gross Unrealized Gains 11 4
Gross Unrealized Losses 2 2
Fair Value 298 289
Debt Securities | Government-sponsored enterprises    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 240 284
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 6
Fair Value 240 278
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
Debt Securities | United States government    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 4 4
Gross Unrealized Gains 0 0
Gross Unrealized Losses 0 0
Fair Value 4 4
Equity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 2,999 2,938
Gross Unrealized Gains 1,987 1,819
Gross Unrealized Losses 45 51
Fair Value 4,941 4,706
Equity Securities | Common equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 2,810 2,749
Gross Unrealized Gains 1,954 1,787
Gross Unrealized Losses 44 51
Fair Value 4,720 4,485
Equity Securities | Nonredeemable preferred equities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 189 189
Gross Unrealized Gains 33 32
Gross Unrealized Losses 1 0
Fair Value 221 221
Fixed Maturities And Equity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Cost or Amortized Cost 12,441 12,262
Gross Unrealized Gains 2,497 2,250
Gross Unrealized Losses 113 156
Fair Value $ 14,825 $ 14,356