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, 2016	Dec. 31, 2015
Investment [Line Items]
Fair Value, 0-12 Months	$ 8,106	$ 16,763
Gross Unrealized Loss, 0-12 Months	(97)	(601)
Fair Value, Over 12 Months	1,501	1,885
Gross Unrealized Loss, Over 12 Months	(76)	(226)
Total Fair Value	9,607	18,648
Total Gross Unrealized Loss	(173)	(827)
U.S. Treasury And Agency [Member]
Investment [Line Items]
Fair Value, 0-12 Months	436	996
Gross Unrealized Loss, 0-12 Months	(2)	(5)
Fair Value, Over 12 Months	0	153
Gross Unrealized Loss, Over 12 Months	0	(1)
Total Fair Value	436	1,149
Total Gross Unrealized Loss	(2)	(6)
Foreign [Member]
Investment [Line Items]
Fair Value, 0-12 Months	1,899	3,953
Gross Unrealized Loss, 0-12 Months	(20)	(148)
Fair Value, Over 12 Months	409	436
Gross Unrealized Loss, Over 12 Months	(28)	(73)
Total Fair Value	2,308	4,389
Total Gross Unrealized Loss	(48)	(221)
Corporate [Member]
Investment [Line Items]
Fair Value, 0-12 Months	1,559	7,518
Gross Unrealized Loss, 0-12 Months	(21)	(371)
Fair Value, Over 12 Months	750	738
Gross Unrealized Loss, Over 12 Months	(45)	(138)
Total Fair Value	2,309	8,256
Total Gross Unrealized Loss	(66)	(509)
Collateralized Mortgage Backed Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	1,039	3,399
Gross Unrealized Loss, 0-12 Months	(3)	(42)
Fair Value, Over 12 Months	292	516
Gross Unrealized Loss, Over 12 Months	(2)	(12)
Total Fair Value	1,331	3,915
Total Gross Unrealized Loss	(5)	(54)
States, Municipalities, And Political Subdivisions [Member]
Investment [Line Items]
Fair Value, 0-12 Months	2,807	556
Gross Unrealized Loss, 0-12 Months	(10)	(6)
Fair Value, Over 12 Months	50	42
Gross Unrealized Loss, Over 12 Months	(1)	(2)
Total Fair Value	2,857	598
Total Gross Unrealized Loss	(11)	(8)
Fixed Maturities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	7,740	16,422
Gross Unrealized Loss, 0-12 Months	(56)	(572)
Fair Value, Over 12 Months	1,501	1,885
Gross Unrealized Loss, Over 12 Months	(76)	(226)
Total Fair Value	9,241	18,307
Total Gross Unrealized Loss	(132)	(798)
Equity Securities [Member]
Investment [Line Items]
Fair Value, 0-12 Months	133	131
Gross Unrealized Loss, 0-12 Months	(14)	(18)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	133	131
Total Gross Unrealized Loss	(14)	(18)
Other Long-term Investments [Member]
Investment [Line Items]
Fair Value, 0-12 Months	233	210
Gross Unrealized Loss, 0-12 Months	(27)	(11)
Fair Value, Over 12 Months	0	0
Gross Unrealized Loss, Over 12 Months	0	0
Total Fair Value	233	210
Total Gross Unrealized Loss	$ (27)	$ (11)