Investments (Aggregate Fair Value And Gross Unrealized Loss By Length Of Time Security Has Continuously Been In Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investment [Line Items]
Fair Value, 0-12 Months	$ 27,692	$ 43,641
Gross Unrealized Loss, 0-12 Months	(375)	(959)
Fair Value, Over 12 Months	1,271	1,310
Gross Unrealized Loss, Over 12 Months	(63)	(78)
Total Fair Value	28,963	44,951
Total Gross Unrealized Loss	(438)	(1,037)
U.S. Treasury And Agency [Member]
Investment [Line Items]
Fair Value, 0-12 Months	2,021	2,216
Gross Unrealized Loss, 0-12 Months	(28)	(48)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	2,021	2,216
Total Gross Unrealized Loss	(28)	(48)
Foreign [Member]
Investment [Line Items]
Fair Value, 0-12 Months	4,381	5,918
Gross Unrealized Loss, 0-12 Months	(57)	(99)
Fair Value, Over 12 Months	599	386
Gross Unrealized Loss, Over 12 Months	(25)	(27)
Total Fair Value	4,980	6,304
Total Gross Unrealized Loss	(82)	(126)
Corporate [Member]
Investment [Line Items]
Fair Value, 0-12 Months	4,410	7,021
Gross Unrealized Loss, 0-12 Months	(72)	(149)
Fair Value, Over 12 Months	372	641
Gross Unrealized Loss, Over 12 Months	(29)	(44)
Total Fair Value	4,782	7,662
Total Gross Unrealized Loss	(101)	(193)
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	7,657	8,638
Gross Unrealized Loss, 0-12 Months	(141)	(189)
Fair Value, Over 12 Months	118	234
Gross Unrealized Loss, Over 12 Months	(4)	(5)
Total Fair Value	7,775	8,872
Total Gross Unrealized Loss	(145)	(194)
States, Municipalities, And Political Subdivisions [Member]
Investment [Line Items]
Fair Value, 0-12 Months	9,049	19,448
Gross Unrealized Loss, 0-12 Months	(68)	(435)
Fair Value, Over 12 Months	182	49
Gross Unrealized Loss, Over 12 Months	(5)	(2)
Total Fair Value	9,231	19,497
Total Gross Unrealized Loss	(73)	(437)
Fixed Maturities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	27,518	43,241
Gross Unrealized Loss, 0-12 Months	(366)	(920)
Fair Value, Over 12 Months	1,271	1,310
Gross Unrealized Loss, Over 12 Months	(63)	(78)
Total Fair Value	28,789	44,551
Total Gross Unrealized Loss	(429)	(998)
Equity Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	104	199
Gross Unrealized Loss, 0-12 Months	(5)	(21)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	104	199
Total Gross Unrealized Loss	(5)	(21)
Other Long-term Investments [Member]
Investment [Line Items]
Fair Value, 0-12 Months	70	201
Gross Unrealized Loss, 0-12 Months	(4)	(18)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	70	201
Total Gross Unrealized Loss	$ (4)	$ (18)