Investments (Aggregate Fair Value And Gross Unrealized Loss By Length Of Time Security Has Continuously Been In Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Fair Value, 0-12 Months	$ 22,243	$ 43,641
Gross Unrealized Loss, 0-12 Months	(287)	(959)
Fair Value, Over 12 Months	3,926	1,310
Gross Unrealized Loss, Over 12 Months	(120)	(78)
Total Fair Value	26,169	44,951
Total Gross Unrealized Loss	(407)	(1,037)
U.S. Treasury And Agency [Member]
Investment [Line Items]
Fair Value, 0-12 Months	1,763	2,216
Gross Unrealized Loss, 0-12 Months	(13)	(48)
Fair Value, Over 12 Months	656	0
Gross Unrealized Loss, Over 12 Months	(17)	0
Total Fair Value	2,419	2,216
Total Gross Unrealized Loss	(30)	(48)
Foreign [Member]
Investment [Line Items]
Fair Value, 0-12 Months	5,353	5,918
Gross Unrealized Loss, 0-12 Months	(85)	(99)
Fair Value, Over 12 Months	894	386
Gross Unrealized Loss, Over 12 Months	(22)	(27)
Total Fair Value	6,247	6,304
Total Gross Unrealized Loss	(107)	(126)
Corporate [Member]
Investment [Line Items]
Fair Value, 0-12 Months	3,373	7,021
Gross Unrealized Loss, 0-12 Months	(48)	(149)
Fair Value, Over 12 Months	788	641
Gross Unrealized Loss, Over 12 Months	(40)	(44)
Total Fair Value	4,161	7,662
Total Gross Unrealized Loss	(88)	(193)
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	6,946	8,638
Gross Unrealized Loss, 0-12 Months	(96)	(189)
Fair Value, Over 12 Months	749	234
Gross Unrealized Loss, Over 12 Months	(19)	(5)
Total Fair Value	7,695	8,872
Total Gross Unrealized Loss	(115)	(194)
States, Municipalities, And Political Subdivisions [Member]
Investment [Line Items]
Fair Value, 0-12 Months	4,548	19,448
Gross Unrealized Loss, 0-12 Months	(28)	(435)
Fair Value, Over 12 Months	839	49
Gross Unrealized Loss, Over 12 Months	(22)	(2)
Total Fair Value	5,387	19,497
Total Gross Unrealized Loss	(50)	(437)
Fixed Maturities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	21,983	43,241
Gross Unrealized Loss, 0-12 Months	(270)	(920)
Fair Value, Over 12 Months	3,926	1,310
Gross Unrealized Loss, Over 12 Months	(120)	(78)
Total Fair Value	25,909	44,551
Total Gross Unrealized Loss	(390)	(998)
Equity Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	103	199
Gross Unrealized Loss, 0-12 Months	(8)	(21)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	103	199
Total Gross Unrealized Loss	(8)	(21)
Other Long-term Investments [Member]
Investment [Line Items]
Fair Value, 0-12 Months	157	201
Gross Unrealized Loss, 0-12 Months	(9)	(18)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	157	201
Total Gross Unrealized Loss	$ (9)	$ (18)