Investments (Aggregate Fair Value And Gross Unrealized Loss By Length Of Time Security Has Continuously Been In Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Fair Value, 0-12 Months		$ 28,685
Gross Unrealized Loss, 0-12 Months		(257)
Fair Value, Over 12 Months		8,885
Gross Unrealized Loss, Over 12 Months		(227)
Total Fair Value		37,570
Total Gross Unrealized Loss		(484)
U.S. Treasury And Agency [Member]
Investment [Line Items]
Fair Value, 0-12 Months	$ 2,869	2,172
Gross Unrealized Loss, 0-12 Months	(55)	(14)
Fair Value, Over 12 Months	1,314	1,249
Gross Unrealized Loss, Over 12 Months	(36)	(26)
Total Fair Value	4,183	3,421
Total Gross Unrealized Loss	(91)	(40)
Foreign [Member]
Investment [Line Items]
Fair Value, 0-12 Months	8,715	5,657
Gross Unrealized Loss, 0-12 Months	(155)	(65)
Fair Value, Over 12 Months	1,643	1,693
Gross Unrealized Loss, Over 12 Months	(52)	(49)
Total Fair Value	10,358	7,350
Total Gross Unrealized Loss	(207)	(114)
Corporate [Member]
Investment [Line Items]
Fair Value, 0-12 Months	12,355	5,210
Gross Unrealized Loss, 0-12 Months	(260)	(56)
Fair Value, Over 12 Months	1,501	1,332
Gross Unrealized Loss, Over 12 Months	(69)	(46)
Total Fair Value	13,856	6,542
Total Gross Unrealized Loss	(329)	(102)
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	12,313	6,194
Gross Unrealized Loss, 0-12 Months	(257)	(31)
Fair Value, Over 12 Months	3,033	3,209
Gross Unrealized Loss, Over 12 Months	(137)	(74)
Total Fair Value	15,346	9,403
Total Gross Unrealized Loss	(394)	(105)
States, Municipalities, And Political Subdivisions [Member]
Investment [Line Items]
Fair Value, 0-12 Months	15,030	9,259
Gross Unrealized Loss, 0-12 Months	(218)	(71)
Fair Value, Over 12 Months	1,343	1,402
Gross Unrealized Loss, Over 12 Months	(51)	(32)
Total Fair Value	16,373	10,661
Total Gross Unrealized Loss	(269)	(103)
Fixed Maturities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	51,282	28,492
Gross Unrealized Loss, 0-12 Months	(945)	(237)
Fair Value, Over 12 Months	8,834	8,885
Gross Unrealized Loss, Over 12 Months	(345)	(227)
Total Fair Value	60,116	37,377
Total Gross Unrealized Loss	$ (1,290)	(464)
Equity Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months		115
Gross Unrealized Loss, 0-12 Months		(12)
Fair Value, Over 12 Months		0
Gross Unrealized Loss, Over 12 Months		0
Total Fair Value		115
Total Gross Unrealized Loss		(12)
Other Long-term Investments [Member]
Investment [Line Items]
Fair Value, 0-12 Months		78
Gross Unrealized Loss, 0-12 Months		(8)
Fair Value, Over 12 Months		0
Gross Unrealized Loss, Over 12 Months		0
Total Fair Value		78
Total Gross Unrealized Loss		$ (8)