Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	$ 431	$ 1,516
Less than 12 months Unrealized Losses	11	74
12 months or more Fair Value	648	86
12 months or more Unrealized Losses	25	5
Total Fair Value	1,079	1,602
Total Unrealized Losses	36	79
Fixed Maturities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	301	1,397
Less than 12 months Unrealized Losses	9	70
12 months or more Fair Value	551	86
12 months or more Unrealized Losses	20	5
Total Fair Value	852	1,483
Total Unrealized Losses	29	75
Fixed Maturities | States, Municipalities And Political Subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	28	490
Less than 12 months Unrealized Losses	0	18
12 months or more Fair Value	218	42
12 months or more Unrealized Losses	3	3
Total Fair Value	246	532
Total Unrealized Losses	3	21
Fixed Maturities | United States Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	1	1
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	1	0
12 months or more Unrealized Losses	0	0
Total Fair Value	2	1
Total Unrealized Losses	0	0
Fixed Maturities | Government-Sponsored Enterprises
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	11	199
Less than 12 months Unrealized Losses	0	27
12 months or more Fair Value	176	1
12 months or more Unrealized Losses	12	0
Total Fair Value	187	200
Total Unrealized Losses	12	27
Fixed Maturities | Foreign Government Debt Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	0	10
Less than 12 months Unrealized Losses	0	0
12 months or more Fair Value	10	0
12 months or more Unrealized Losses	0	0
Total Fair Value	10	10
Total Unrealized Losses	0	0
Fixed Maturities | Commercial Mortgage Backed Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	37	125
Less than 12 months Unrealized Losses	0	5
12 months or more Fair Value	32	0
12 months or more Unrealized Losses	1	0
Total Fair Value	69	125
Total Unrealized Losses	1	5
Fixed Maturities | Corporate Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	224	572
Less than 12 months Unrealized Losses	9	20
12 months or more Fair Value	114	43
12 months or more Unrealized Losses	4	2
Total Fair Value	338	615
Total Unrealized Losses	13	22
Equity Securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	130	119
Less than 12 months Unrealized Losses	2	4
12 months or more Fair Value	97	0
12 months or more Unrealized Losses	5	0
Total Fair Value	227	119
Total Unrealized Losses	7	4
Equity Securities | Common Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	105	77
Less than 12 months Unrealized Losses	2	1
12 months or more Fair Value	80	0
12 months or more Unrealized Losses	4	0
Total Fair Value	185	77
Total Unrealized Losses	6	1
Equity Securities | Preferred Stock
Investments, Unrealized Loss Position [Line Items]
Less than 12 months Fair Value	25	42
Less than 12 months Unrealized Losses	0	3
12 months or more Fair Value	17	0
12 months or more Unrealized Losses	1	0
Total Fair Value	42	42
Total Unrealized Losses	$ 1	$ 3