Investments - Securities Available-for-Sale in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		$ 3,620
Gross Unrealized Losses, Less than 12 Months		40
Estimated Fair Value, 12 Months or Longer		1,455
Gross Unrealized Losses, 12 Months or Longer		49
Total Estimated Fair Value		5,075
Total Gross Unrealized Losses		89
Corporate Bonds and Other [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	$ 6,963	1,354
Gross Unrealized Losses, Less than 12 Months	192	21
Estimated Fair Value, 12 Months or Longer	193	168
Gross Unrealized Losses, 12 Months or Longer	11	7
Total Estimated Fair Value	7,156	1,522
Total Gross Unrealized Losses	203	28
States, Municipalities and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	723	72
Gross Unrealized Losses, Less than 12 Months	8	1
Estimated Fair Value, 12 Months or Longer	3	85
Gross Unrealized Losses, 12 Months or Longer		1
Total Estimated Fair Value	726	157
Total Gross Unrealized Losses	8	2
Residential Mortgage-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	3,183	1,228
Gross Unrealized Losses, Less than 12 Months	68	5
Estimated Fair Value, 12 Months or Longer	344	947
Gross Unrealized Losses, 12 Months or Longer	17	27
Total Estimated Fair Value	3,527	2,175
Total Gross Unrealized Losses	85	32
Commercial Mortgage-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	913	403
Gross Unrealized Losses, Less than 12 Months	16	4
Estimated Fair Value, 12 Months or Longer	228	212
Gross Unrealized Losses, 12 Months or Longer	17	10
Total Estimated Fair Value	1,141	615
Total Gross Unrealized Losses	33	14
Other Asset-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	632	248
Gross Unrealized Losses, Less than 12 Months	6	3
Estimated Fair Value, 12 Months or Longer	27	18
Gross Unrealized Losses, 12 Months or Longer	3	2
Total Estimated Fair Value	659	266
Total Gross Unrealized Losses	9	5
Total Asset-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	4,728	1,879
Gross Unrealized Losses, Less than 12 Months	90	12
Estimated Fair Value, 12 Months or Longer	599	1,177
Gross Unrealized Losses, 12 Months or Longer	37	39
Total Estimated Fair Value	5,327	3,056
Total Gross Unrealized Losses	127	51
U.S. Treasury and Obligations of Government-Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	49	49
Gross Unrealized Losses, Less than 12 Months	1	2
Estimated Fair Value, 12 Months or Longer	19	21
Gross Unrealized Losses, 12 Months or Longer	2	2
Total Estimated Fair Value	68	70
Total Gross Unrealized Losses	3	4
Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	178	166
Gross Unrealized Losses, Less than 12 Months	3	2
Estimated Fair Value, 12 Months or Longer	40	4
Gross Unrealized Losses, 12 Months or Longer	2
Total Estimated Fair Value	218	170
Total Gross Unrealized Losses	5	2
Fixed Maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	12,641	3,520
Gross Unrealized Losses, Less than 12 Months	294	38
Estimated Fair Value, 12 Months or Longer	854	1,455
Gross Unrealized Losses, 12 Months or Longer	52	49
Total Estimated Fair Value	13,495	4,975
Total Gross Unrealized Losses	$ 346	87
Common Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		7
Gross Unrealized Losses, Less than 12 Months		1
Total Estimated Fair Value		7
Total Gross Unrealized Losses		1
Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		93
Gross Unrealized Losses, Less than 12 Months		1
Total Estimated Fair Value		93
Total Gross Unrealized Losses		1
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		100
Gross Unrealized Losses, Less than 12 Months		2
Total Estimated Fair Value		100
Total Gross Unrealized Losses		$ 2