Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 844	$ 1,654
Less than 12 months Unrealized Losses	74	119
12 months or more Fair Value	286	284
12 months or more Unrealized Losses	18	37
Total Fair Value	1,130	1,938
Total Unrealized Losses	92	156
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	279	1,349
Less than 12 months Unrealized Losses	9	68
12 months or more Fair Value	268	284
12 months or more Unrealized Losses	18	37
Total Fair Value	547	1,633
Total Unrealized Losses	27	105
Fixed Maturities | Corporate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	252	1,099
Less than 12 months Unrealized Losses	9	63
12 months or more Fair Value	258	133
12 months or more Unrealized Losses	18	33
Total Fair Value	510	1,232
Total Unrealized Losses	27	96
Fixed Maturities | States, municipalities and political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	2	47
Less than 12 months Unrealized Losses	0	1
12 months or more Fair Value	1	22
12 months or more Unrealized Losses	0	0
Total Fair Value	3	69
Total Unrealized Losses	0	1
Fixed Maturities | Commercial mortgage-backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	5	103
Less than 12 months Unrealized Losses	0	2
12 months or more Fair Value	9	2
12 months or more Unrealized Losses	0	0
Total Fair Value	14	105
Total Unrealized Losses	0	2
Fixed Maturities | Government-sponsored enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	20	100
Less than 12 months Unrealized Losses	0	2
12 months or more Fair Value	0	127
12 months or more Unrealized Losses	0	4
Total Fair Value	20	227
Total Unrealized Losses	0	6
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	565	305
Less than 12 months Unrealized Losses	65	51
12 months or more Fair Value	18	0
12 months or more Unrealized Losses	0	0
Total Fair Value	583	305
Total Unrealized Losses	65	51
Equity Securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	546	270
Less than 12 months Unrealized Losses	65	51
12 months or more Fair Value	0	0
12 months or more Unrealized Losses	0	0
Total Fair Value	546	270
Total Unrealized Losses	65	51
Equity Securities | Nonredeemable preferred equities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	19	35
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	18	0
12 months or more Unrealized Losses	0	0
Total Fair Value	37	35
Total Unrealized Losses	$ 0	$ 0