Investments - Securities Available-for-Sale in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	$ 2,148	$ 12,689
Gross Unrealized Losses, Less than 12 Months	31	404
Estimated Fair Value, 12 Months or Longer	272	2,614
Gross Unrealized Losses, 12 Months or Longer	14	117
Total Estimated Fair Value	2,420	15,303
Total Gross Unrealized Losses	45	521
Corporate Bonds and Other [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	914	8,543
Gross Unrealized Losses, Less than 12 Months	21	340
Estimated Fair Value, 12 Months or Longer	186	825
Gross Unrealized Losses, 12 Months or Longer	11	55
Total Estimated Fair Value	1,100	9,368
Total Gross Unrealized Losses	32	395
States, Municipalities and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	34	517
Gross Unrealized Losses, Less than 12 Months		8
Estimated Fair Value, 12 Months or Longer		5
Gross Unrealized Losses, 12 Months or Longer		1
Total Estimated Fair Value	34	522
Total Gross Unrealized Losses		9
Residential Mortgage-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	249	1,932
Gross Unrealized Losses, Less than 12 Months	1	23
Estimated Fair Value, 12 Months or Longer	30	1,119
Gross Unrealized Losses, 12 Months or Longer		34
Total Estimated Fair Value	279	3,051
Total Gross Unrealized Losses	1	57
Commercial Mortgage-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	381	728
Gross Unrealized Losses, Less than 12 Months	3	10
Estimated Fair Value, 12 Months or Longer	20	397
Gross Unrealized Losses, 12 Months or Longer	2	22
Total Estimated Fair Value	401	1,125
Total Gross Unrealized Losses	5	32
Other Asset-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	449	834
Gross Unrealized Losses, Less than 12 Months	3	21
Estimated Fair Value, 12 Months or Longer	33	125
Gross Unrealized Losses, 12 Months or Longer	1	3
Total Estimated Fair Value	482	959
Total Gross Unrealized Losses	4	24
Total Asset-Backed [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	1,079	3,494
Gross Unrealized Losses, Less than 12 Months	7	54
Estimated Fair Value, 12 Months or Longer	83	1,641
Gross Unrealized Losses, 12 Months or Longer	3	59
Total Estimated Fair Value	1,162	5,135
Total Gross Unrealized Losses	10	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	62	21
Gross Unrealized Losses, Less than 12 Months	2
Estimated Fair Value, 12 Months or Longer	2	19
Total Estimated Fair Value	64	40
Total Gross Unrealized Losses	2
Foreign Government [Member]
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value, Less than 12 Months	59	114
Gross Unrealized Losses, Less than 12 Months	1	2
Estimated Fair Value, 12 Months or Longer	1	124
Gross Unrealized Losses, 12 Months or Longer		2
Total Estimated Fair Value	60	238
Total Gross Unrealized Losses	$ 1	$ 4