Investments - Securities Available-for-Sale in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Corporate and Other Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 1,216	$ 2,615
Gross Unrealized Losses, Less than 12 Months	21	61
Estimated Fair Value, 12 Months or Longer	91	254
Gross Unrealized Losses, 12 Months or Longer	5	15
Total Estimated Fair Value	1,307	2,869
Total Gross Unrealized Losses	26	76
States, Municipalities and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	583	959
Gross Unrealized Losses, Less than 12 Months	6	32
Estimated Fair Value, 12 Months or Longer	56	23
Gross Unrealized Losses, 12 Months or Longer	1	1
Total Estimated Fair Value	639	982
Total Gross Unrealized Losses	7	33
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	1,522	2,136
Gross Unrealized Losses, Less than 12 Months	25	44
Estimated Fair Value, 12 Months or Longer	106	201
Gross Unrealized Losses, 12 Months or Longer	3	7
Total Estimated Fair Value	1,628	2,337
Total Gross Unrealized Losses	28	51
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	378	756
Gross Unrealized Losses, Less than 12 Months	6	22
Estimated Fair Value, 12 Months or Longer	138	69
Gross Unrealized Losses, 12 Months or Longer	7	2
Total Estimated Fair Value	516	825
Total Gross Unrealized Losses	13	24
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	129	398
Gross Unrealized Losses, Less than 12 Months	4	5
Estimated Fair Value, 12 Months or Longer	10	24
Total Estimated Fair Value	139	422
Total Gross Unrealized Losses	4	5
Total Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2,029	3,290
Gross Unrealized Losses, Less than 12 Months	35	71
Estimated Fair Value, 12 Months or Longer	254	294
Gross Unrealized Losses, 12 Months or Longer	10	9
Total Estimated Fair Value	2,283	3,584
Total Gross Unrealized Losses	45	80
U.S. Treasury and Obligations of Government-Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	67	5
Gross Unrealized Losses, Less than 12 Months	3
Estimated Fair Value, 12 Months or Longer	6
Total Estimated Fair Value	73	5
Total Gross Unrealized Losses	3
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	191	108
Gross Unrealized Losses, Less than 12 Months	4	3
Estimated Fair Value, 12 Months or Longer	5
Total Estimated Fair Value	196	108
Total Gross Unrealized Losses	4	3
Total Fixed Maturities Available-for-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	4,086	6,977
Gross Unrealized Losses, Less than 12 Months	69	167
Estimated Fair Value, 12 Months or Longer	412	571
Gross Unrealized Losses, 12 Months or Longer	16	25
Total Estimated Fair Value	4,498	7,548
Total Gross Unrealized Losses	85	192
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	18	12
Gross Unrealized Losses, Less than 12 Months	1
Estimated Fair Value, 12 Months or Longer		13
Gross Unrealized Losses, 12 Months or Longer		4
Total Estimated Fair Value	18	25
Total Gross Unrealized Losses	1	4
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	4,104	6,989
Gross Unrealized Losses, Less than 12 Months	70	167
Estimated Fair Value, 12 Months or Longer	412	584
Gross Unrealized Losses, 12 Months or Longer	16	29
Total Estimated Fair Value	4,516	7,573
Total Gross Unrealized Losses	86	$ 196
Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2
Gross Unrealized Losses, Less than 12 Months	1
Total Estimated Fair Value	2
Total Gross Unrealized Losses	1
Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	16
Total Estimated Fair Value	$ 16