Investments - Securities Available-for-Sale in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Corporate Bonds and Other [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	$ 776	$ 8,543
Gross Unrealized Losses, Less than 12 Months	22	340
Estimated Fair Value, 12 Months or Longer	498	825
Gross Unrealized Losses, 12 Months or Longer	22	55
Total Estimated Fair Value	1,274	9,368
Total Gross Unrealized Losses	44	395
States, Municipalities and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	19	517
Gross Unrealized Losses, Less than 12 Months		8
Estimated Fair Value, 12 Months or Longer	2	5
Gross Unrealized Losses, 12 Months or Longer		1
Total Estimated Fair Value	21	522
Total Gross Unrealized Losses		9
Residential Mortgage-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	163	1,932
Gross Unrealized Losses, Less than 12 Months		23
Estimated Fair Value, 12 Months or Longer	134	1,119
Gross Unrealized Losses, 12 Months or Longer	2	34
Total Estimated Fair Value	297	3,051
Total Gross Unrealized Losses	2	57
Commercial Mortgage-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	58	728
Gross Unrealized Losses, Less than 12 Months	2	10
Estimated Fair Value, 12 Months or Longer	69	397
Gross Unrealized Losses, 12 Months or Longer	2	22
Total Estimated Fair Value	127	1,125
Total Gross Unrealized Losses	4	32
Other Asset-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	386	834
Gross Unrealized Losses, Less than 12 Months	5	21
Estimated Fair Value, 12 Months or Longer	77	125
Gross Unrealized Losses, 12 Months or Longer	2	3
Total Estimated Fair Value	463	959
Total Gross Unrealized Losses	7	24
Total Asset-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	607	3,494
Gross Unrealized Losses, Less than 12 Months	7	54
Estimated Fair Value, 12 Months or Longer	280	1,641
Gross Unrealized Losses, 12 Months or Longer	6	59
Total Estimated Fair Value	887	5,135
Total Gross Unrealized Losses	13	113
U.S. Treasury and obligations of government-sponsored enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months		21
Estimated Fair Value, 12 Months or Longer	4	19
Total Estimated Fair Value	4	40
Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	3	114
Gross Unrealized Losses, Less than 12 Months		2
Estimated Fair Value, 12 Months or Longer	11	124
Gross Unrealized Losses, 12 Months or Longer		2
Total Estimated Fair Value	14	238
Total Gross Unrealized Losses		4
Fixed Maturity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	1,405	12,689
Gross Unrealized Losses, Less than 12 Months	29	404
Estimated Fair Value, 12 Months or Longer	795	2,614
Gross Unrealized Losses, 12 Months or Longer	28	117
Total Estimated Fair Value	2,200	15,303
Total Gross Unrealized Losses	$ 57	$ 521