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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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