Investments - Securities Available-for-Sale in Gross Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Corporate and Other Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	$ 1,915	$ 2,615
Gross Unrealized Losses, Less than 12 Months	29	61
Estimated Fair Value, 12 Months or Longer	134	254
Gross Unrealized Losses, 12 Months or Longer	8	15
Total Estimated Fair Value	2,049	2,869
Total Gross Unrealized Losses	37	76
States, Municipalities and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	845	959
Gross Unrealized Losses, Less than 12 Months	31	32
Estimated Fair Value, 12 Months or Longer	33	23
Gross Unrealized Losses, 12 Months or Longer		1
Total Estimated Fair Value	878	982
Total Gross Unrealized Losses	31	33
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2,188	2,136
Gross Unrealized Losses, Less than 12 Months	44	44
Estimated Fair Value, 12 Months or Longer	170	201
Gross Unrealized Losses, 12 Months or Longer	6	7
Total Estimated Fair Value	2,358	2,337
Total Gross Unrealized Losses	50	51
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	590	756
Gross Unrealized Losses, Less than 12 Months	15	22
Estimated Fair Value, 12 Months or Longer	139	69
Gross Unrealized Losses, 12 Months or Longer	4	2
Total Estimated Fair Value	729	825
Total Gross Unrealized Losses	19	24
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	256	398
Gross Unrealized Losses, Less than 12 Months	4	5
Estimated Fair Value, 12 Months or Longer	28	24
Total Estimated Fair Value	284	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	3,034	3,290
Gross Unrealized Losses, Less than 12 Months	63	71
Estimated Fair Value, 12 Months or Longer	337	294
Gross Unrealized Losses, 12 Months or Longer	10	9
Total Estimated Fair Value	3,371	3,584
Total Gross Unrealized Losses	73	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	31	5
Total Estimated Fair Value	31	5
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	89	108
Gross Unrealized Losses, Less than 12 Months	1	3
Total Estimated Fair Value	89	108
Total Gross Unrealized Losses	1	3
Total Fixed Maturities Available-for-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	5,914	6,977
Gross Unrealized Losses, Less than 12 Months	124	167
Estimated Fair Value, 12 Months or Longer	504	571
Gross Unrealized Losses, 12 Months or Longer	18	25
Total Estimated Fair Value	6,418	7,548
Total Gross Unrealized Losses	142	192
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months		12
Estimated Fair Value, 12 Months or Longer		13
Gross Unrealized Losses, 12 Months or Longer		4
Total Estimated Fair Value		25
Total Gross Unrealized Losses		4
Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	5,931	6,989
Gross Unrealized Losses, Less than 12 Months	126	167
Estimated Fair Value, 12 Months or Longer	504	584
Gross Unrealized Losses, 12 Months or Longer	18	29
Total Estimated Fair Value	6,435	7,573
Total Gross Unrealized Losses	144	$ 196
Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	2
Total Estimated Fair Value	2
Preferred Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Estimated Fair Value, Less than 12 Months	15
Gross Unrealized Losses, Less than 12 Months	2
Total Estimated Fair Value	15
Total Gross Unrealized Losses	$ 2