Investments (Aggregate Fair Value And Gross Unrealized Loss By Length Of Time Security Has Continuously Been In Unrealized Loss Position) (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		$ 28,685
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(257)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		8,885
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(227)
Investment securities, Continuous Unrealized loss position, Total Fair Value		37,570
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(484)
Fixed Maturities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 34,952
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(953)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	27,762
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(817)
Debt Securities, Available-for-sale, Unrealized Loss Position	62,714
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(1,770)
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		28,492
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(237)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		8,885
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(227)
Investment securities, Continuous Unrealized loss position, Total Fair Value		37,377
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(464)
Equity securities
Investment [Line Items]
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		115
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(12)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		0
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		0
Investment securities, Continuous Unrealized loss position, Total Fair Value		115
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(12)
U.S. Treasury and agency | Fixed Maturities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	523
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(4)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	2,859
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(50)
Debt Securities, Available-for-sale, Unrealized Loss Position	3,382
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(54)
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		2,172
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(14)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		1,249
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(26)
Investment securities, Continuous Unrealized loss position, Total Fair Value		3,421
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(40)
Foreign | Fixed Maturities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	6,764
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(208)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,349
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(159)
Debt Securities, Available-for-sale, Unrealized Loss Position	12,113
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(367)
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		5,657
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(65)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		1,693
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(49)
Investment securities, Continuous Unrealized loss position, Total Fair Value		7,350
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(114)
Corporate securities | Fixed Maturities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	16,538
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(599)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	4,873
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(255)
Debt Securities, Available-for-sale, Unrealized Loss Position	21,411
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(854)
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		5,210
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(56)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		1,332
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(46)
Investment securities, Continuous Unrealized loss position, Total Fair Value		6,542
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(102)
Mortgage-backed securities | Fixed Maturities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	6,103
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(98)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	6,913
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(229)
Debt Securities, Available-for-sale, Unrealized Loss Position	13,016
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(327)
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		6,194
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(31)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		3,209
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(74)
Investment securities, Continuous Unrealized loss position, Total Fair Value		9,403
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(105)
States, municipalities, and political subdivisions | Fixed Maturities [Member]
Investment [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	5,024
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(44)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	7,768
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(124)
Debt Securities, Available-for-sale, Unrealized Loss Position	12,792
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	$ (168)
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		9,259
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(71)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		1,402
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		(32)
Investment securities, Continuous Unrealized loss position, Total Fair Value		10,661
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		(103)
Other investments
Investment [Line Items]
Investment securities, Continuous Unrealized loss position, 0-12 Months, Fair Value		78
Investment Securities, Continuous Unrealized Loss position, Less than 12 Months, Accumulated Loss		(8)
Investment securities, Continuous Unrealized loss position, Over 12 Months, Fair Value		0
Investment securities, Continuous Unrealized loss position, Over 12 Months, Gross Unrealized Loss		0
Investment securities, Continuous Unrealized loss position, Total Fair Value		78
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ (8)