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Investments - Amortized Cost and Fair Values of Securities (Detail) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
--- | --- | ---
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | $ 39,822 | $ 39,518
Gross Unrealized Gains | 3,461 | 2,807
Gross Unrealized Losses | 166 | 282
Estimated Fair Value | 43,117 | 42,043
Unrealized OTTI Losses (Gains) | (42) | (45)
Fixed Maturity Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 39,230 | 38,947
Gross Unrealized Gains | 3,363 | 2,739
Gross Unrealized Losses | 86 | 192
Estimated Fair Value | 42,507 | 41,494
Unrealized OTTI Losses (Gains) | (42) | (45)
Corporate and Other Bonds [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 17,965 | 17,711
Gross Unrealized Gains | 1,645 | 1,323
Gross Unrealized Losses | 26 | 76
Estimated Fair Value | 19,584 | 18,958
Unrealized OTTI Losses (Gains) |  | (1)
States, Municipalities and Political Subdivisions [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 12,462 | 12,060
Gross Unrealized Gains | 1,501 | 1,213
Gross Unrealized Losses | 7 | 33
Estimated Fair Value | 13,956 | 13,240
Unrealized OTTI Losses (Gains) | (14) | (16)
Residential Mortgage-Backed [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 4,906 | 5,004
Gross Unrealized Gains | 127 | 120
Gross Unrealized Losses | 28 | 51
Estimated Fair Value | 5,005 | 5,073
Unrealized OTTI Losses (Gains) | (28) | (28)
Commercial Mortgage-Backed [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 1,858 | 2,016
Gross Unrealized Gains | 55 | 48
Gross Unrealized Losses | 13 | 24
Estimated Fair Value | 1,900 | 2,040
Other Asset-Backed [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 1,047 | 1,022
Gross Unrealized Gains | 18 | 8
Gross Unrealized Losses | 4 | 5
Estimated Fair Value | 1,061 | 1,025
Total Asset-Backed [Member] | Fixed Maturity Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 7,811 | 8,042
Gross Unrealized Gains | 200 | 176
Gross Unrealized Losses | 45 | 80
Estimated Fair Value | 7,966 | 8,138
Unrealized OTTI Losses (Gains) | (28) | (28)
U.S. Treasury and Obligations of Government-Sponsored Enterprises [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 115 | 83
Gross Unrealized Gains | 3 | 10
Gross Unrealized Losses | 3 |
Estimated Fair Value | 115 | 93
Foreign Government [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 439 | 435
Gross Unrealized Gains | 10 | 13
Gross Unrealized Losses | 4 | 3
Estimated Fair Value | 445 | 445
Redeemable Preferred Stock [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 18 | 18
Gross Unrealized Gains | 2 | 1
Estimated Fair Value | 20 | 19
Total Fixed Maturities Available-for-Sale [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 38,810 | 38,349
Gross Unrealized Gains | 3,361 | 2,736
Gross Unrealized Losses | 85 | 192
Estimated Fair Value | 42,086 | 40,893
Unrealized OTTI Losses (Gains) | (42) | (45)
Fixed Maturities Trading [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 420 | 598
Gross Unrealized Gains | 2 | 3
Gross Unrealized Losses | 1 |
Estimated Fair Value | 421 | 601
Common Stock [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 16 | 13
Gross Unrealized Gains | 7 | 6
Gross Unrealized Losses | 1 |
Estimated Fair Value | 22 | 19
Preferred Stock [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 102 | 93
Gross Unrealized Gains | 5 | 2
Gross Unrealized Losses |  | 4
Estimated Fair Value | 107 | 91
Equity Securities Available-for-Sale [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 118 | 106
Gross Unrealized Gains | 12 | 8
Gross Unrealized Losses | 1 | 4
Estimated Fair Value | 129 | 110
Equity Securities Trading [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 474 | 465
Gross Unrealized Gains | 86 | 60
Gross Unrealized Losses | 79 | 86
Estimated Fair Value | 481 | 439
Equity Securities [Member] |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Cost or Amortized Cost | 592 | 571
Gross Unrealized Gains | 98 | 68
Gross Unrealized Losses | 80 | 90
Estimated Fair Value | $ 610 | $ 549

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