Investments - Amortized Cost and Fair Values of Fixed Maturity and Equity Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	$ 38,568	$ 40,038
Gross Unrealized Gains	2,220	3,491
Gross Unrealized Losses	347	172
Estimated Fair Value	40,441	43,357
Unrealized OTTI Losses (Gains)	(25)	(38)
Corporate Bonds and Other [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	17,883	17,210
Gross Unrealized Gains	938	1,625
Gross Unrealized Losses	203	28
Estimated Fair Value	18,618	18,807
States, Municipalities and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	10,856	12,478
Gross Unrealized Gains	1,161	1,551
Gross Unrealized Losses	8	2
Estimated Fair Value	12,009	14,027
Unrealized OTTI Losses (Gains)		(11)
Residential Mortgage-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	4,961	5,043
Gross Unrealized Gains	72	109
Gross Unrealized Losses	85	32
Estimated Fair Value	4,948	5,120
Unrealized OTTI Losses (Gains)	(25)	(27)
Commercial Mortgage-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	2,082	1,840
Gross Unrealized Gains	23	46
Gross Unrealized Losses	33	14
Estimated Fair Value	2,072	1,872
Other Asset-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	1,557	1,083
Gross Unrealized Gains	9	16
Gross Unrealized Losses	9	5
Estimated Fair Value	1,557	1,094
Total Asset-Backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	8,600	7,966
Gross Unrealized Gains	104	171
Gross Unrealized Losses	127	51
Estimated Fair Value	8,577	8,086
Unrealized OTTI Losses (Gains)	(25)	(27)
U.S. Treasury and Obligations of Government-Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	127	111
Gross Unrealized Gains	3	2
Gross Unrealized Losses	3	4
Estimated Fair Value	127	109
Foreign Government [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	436	437
Gross Unrealized Gains	6	9
Gross Unrealized Losses	5	2
Estimated Fair Value	437	444
Redeemable Preferred Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	9	10
Gross Unrealized Gains	1	1
Estimated Fair Value	10	11
Total Fixed Maturities Available-for-Sale [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	37,911	38,212
Gross Unrealized Gains	2,213	3,359
Gross Unrealized Losses	346	87
Estimated Fair Value	39,778	41,484
Unrealized OTTI Losses (Gains)	(25)	(38)
Fixed Maturities Trading [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	657	649
Gross Unrealized Gains	7	2
Gross Unrealized Losses	1	2
Estimated Fair Value	$ 663	649
Fixed Maturity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost		38,861
Gross Unrealized Gains		3,361
Gross Unrealized Losses		89
Estimated Fair Value		42,133
Unrealized OTTI Losses (Gains)		(38)
Common Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost		21
Gross Unrealized Gains		7
Gross Unrealized Losses		1
Estimated Fair Value		27
Preferred Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost		638
Gross Unrealized Gains		31
Gross Unrealized Losses		1
Estimated Fair Value		668
Equity Securities Available-for-Sale [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost		659
Gross Unrealized Gains		38
Gross Unrealized Losses		2
Estimated Fair Value		695
Equity Securities Trading [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost		518
Gross Unrealized Gains		92
Gross Unrealized Losses		81
Estimated Fair Value		529
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost		1,177
Gross Unrealized Gains		130
Gross Unrealized Losses		83
Estimated Fair Value		$ 1,224