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, 2017	Dec. 31, 2016
Investment [Line Items]
Fair Value, 0-12 Months	$ 38,278	$ 43,641
Gross Unrealized Loss, 0-12 Months	(678)	(959)
Fair Value, Over 12 Months	1,429	1,310
Gross Unrealized Loss, Over 12 Months	(60)	(78)
Total Fair Value	39,707	44,951
Total Gross Unrealized Loss	(738)	(1,037)
U.S. Treasury And Agency [Member]
Investment [Line Items]
Fair Value, 0-12 Months	2,011	2,216
Gross Unrealized Loss, 0-12 Months	(38)	(48)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	2,011	2,216
Total Gross Unrealized Loss	(38)	(48)
Foreign [Member]
Investment [Line Items]
Fair Value, 0-12 Months	4,428	5,918
Gross Unrealized Loss, 0-12 Months	(59)	(99)
Fair Value, Over 12 Months	684	386
Gross Unrealized Loss, Over 12 Months	(21)	(27)
Total Fair Value	5,112	6,304
Total Gross Unrealized Loss	(80)	(126)
Corporate [Member]
Investment [Line Items]
Fair Value, 0-12 Months	6,139	7,021
Gross Unrealized Loss, 0-12 Months	(121)	(149)
Fair Value, Over 12 Months	491	641
Gross Unrealized Loss, Over 12 Months	(32)	(44)
Total Fair Value	6,630	7,662
Total Gross Unrealized Loss	(153)	(193)
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	8,735	8,638
Gross Unrealized Loss, 0-12 Months	(195)	(189)
Fair Value, Over 12 Months	131	234
Gross Unrealized Loss, Over 12 Months	(3)	(5)
Total Fair Value	8,866	8,872
Total Gross Unrealized Loss	(198)	(194)
States, Municipalities, And Political Subdivisions [Member]
Investment [Line Items]
Fair Value, 0-12 Months	16,731	19,448
Gross Unrealized Loss, 0-12 Months	(248)	(435)
Fair Value, Over 12 Months	123	49
Gross Unrealized Loss, Over 12 Months	(4)	(2)
Total Fair Value	16,854	19,497
Total Gross Unrealized Loss	(252)	(437)
Fixed Maturities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	38,044	43,241
Gross Unrealized Loss, 0-12 Months	(661)	(920)
Fair Value, Over 12 Months	1,429	1,310
Gross Unrealized Loss, Over 12 Months	(60)	(78)
Total Fair Value	39,473	44,551
Total Gross Unrealized Loss	(721)	(998)
Equity Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	150	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	150	199
Total Gross Unrealized Loss	(8)	(21)
Other Long-term Investments [Member]
Investment [Line Items]
Fair Value, 0-12 Months	84	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	84	201
Total Gross Unrealized Loss	$ (9)	$ (18)