Investments and Cash - Fixed Maturity Securities and Short Term Investments by Security Type (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Investments
Percent of Total	100.00%	[1],[2]	100.00%	[1],[2]
Amortized Cost, total investment portfolio	$ 10,449	[2]	$ 10,392	[2]
Gross Unrealized Gains, total investment portfolio	455	[2]	370	[2]
Gross Unrealized Losses, total investment portfolio	(90)	[2]	(147)	[2]
Estimated Fair Value, total investment portfolio	10,814	[2]	10,615	[2]
AOCI Gain (Loss) on Securities with OTTI	(21)	[2],[3]	(37)	[2]
Percentage of mortgage backed securities representing government-agency obligations	45.00%		50.00%
Fixed maturity securities
Investments
Percent of Total	93.00%	[1],[2]	91.00%	[1],[2]
Available-for-sale Securities, Amortized Cost Basis	9,729	[2]	9,488	[2]
Gross Unrealized Gains, total fixed maturity securities	455	[2]	370	[2]
Gross Unrealized Losses, total fixed maturity securities	(90)	[2]	(147)	[2]
Estimated Fair Value, total fixed maturity securities	10,094	[2]	9,711	[2]
AOCI Gain (Loss) on Securities with OTTI	(21)	[2],[3]	(37)	[2]
U.S. government and agencies
Investments
Percent of Total	7.00%	[1],[2]	7.00%	[1],[2]
Available-for-sale Securities, Amortized Cost Basis	674	[2]	674	[2]
Gross Unrealized Gains, total fixed maturity securities	32	[2]	32	[2]
Gross Unrealized Losses, total fixed maturity securities	(5)	[2]	(6)	[2]
Estimated Fair Value, total fixed maturity securities	701	[2]	700	[2]
AOCI Gain (Loss) on Securities with OTTI	0	[2],[3]	0	[2]
Obligations of state and political subdivisions
Investments
Percent of Total	48.00%	[1],[2]	47.00%	[1],[2]
Available-for-sale Securities, Amortized Cost Basis	4,982	[2]	4,899	[2]
Gross Unrealized Gains, total fixed maturity securities	281	[2]	219	[2]
Gross Unrealized Losses, total fixed maturity securities	(14)	[2]	(39)	[2]
Estimated Fair Value, total fixed maturity securities	5,249	[2]	5,079	[2]
AOCI Gain (Loss) on Securities with OTTI	7	[2],[3]	4	[2]
Corporate securities
Investments
Percent of Total	13.00%	[1],[2]	13.00%	[1],[2]
Available-for-sale Securities, Amortized Cost Basis	1,382	[2]	1,314	[2]
Gross Unrealized Gains, total fixed maturity securities	55	[2]	44	[2]
Gross Unrealized Losses, total fixed maturity securities	(10)	[2]	(18)	[2]
Estimated Fair Value, total fixed maturity securities	1,427	[2]	1,340	[2]
AOCI Gain (Loss) on Securities with OTTI	0	[2],[3]	0	[2]
RMBS
Investments
Percent of Total	11.00%	[1],[2],[4]	11.00%	[1],[2],[4]
Available-for-sale Securities, Amortized Cost Basis	1,182	[2],[4]	1,160	[2],[4]
Gross Unrealized Gains, total fixed maturity securities	44	[2],[4]	34	[2],[4]
Gross Unrealized Losses, total fixed maturity securities	(55)	[2],[4]	(72)	[2],[4]
Estimated Fair Value, total fixed maturity securities	1,171	[2],[4]	1,122	[2],[4]
AOCI Gain (Loss) on Securities with OTTI	(31)	[2],[3],[4]	(43)	[2],[4]
CMBS
Investments
Percent of Total	6.00%	[1],[2],[4]	5.00%	[1],[2],[4]
Available-for-sale Securities, Amortized Cost Basis	656	[2],[4]	536	[2],[4]
Gross Unrealized Gains, total fixed maturity securities	16	[2],[4]	17	[2],[4]
Gross Unrealized Losses, total fixed maturity securities	(2)	[2],[4]	(4)	[2],[4]
Estimated Fair Value, total fixed maturity securities	670	[2],[4]	549	[2],[4]
AOCI Gain (Loss) on Securities with OTTI	0	[2],[3],[4]	0	[2],[4]
Asset-backed securities
Investments
Percent of Total	5.00%	[1],[2]	6.00%	[1],[2]
Available-for-sale Securities, Amortized Cost Basis	544	[2]	605	[2]
Gross Unrealized Gains, total fixed maturity securities	13	[2]	10	[2]
Gross Unrealized Losses, total fixed maturity securities	(3)	[2]	(7)	[2]
Estimated Fair Value, total fixed maturity securities	554	[2]	608	[2]
AOCI Gain (Loss) on Securities with OTTI	3	[2],[3]	2	[2]
Foreign government securities
Investments
Percent of Total	3.00%	[1],[2]	3.00%	[1],[2]
Available-for-sale Securities, Amortized Cost Basis	309	[2]	300	[2]
Gross Unrealized Gains, total fixed maturity securities	14	[2]	14	[2]
Gross Unrealized Losses, total fixed maturity securities	(1)	[2]	(1)	[2]
Estimated Fair Value, total fixed maturity securities	322	[2]	313	[2]
AOCI Gain (Loss) on Securities with OTTI	0	[2],[3]	0	[2]
Short-term investments
Investments
Percent of Total	7.00%	[1],[2]	9.00%	[1],[2]
Amortized Cost, total investment portfolio	720	[2]	904	[2]
Gross Unrealized Gains, total investment portfolio	0	[2]	0	[2]
Gross Unrealized Losses, total investment portfolio	0	[2]	0	[2]
Estimated Fair Value, total investment portfolio	720	[2]	904	[2]
AOCI Gain (Loss) on Securities with OTTI	$ 0	[2],[3]	$ 0	[2]

[1] Based on amortized cost.

[2] Ratings in the tables above represent the lower of the Moody’s and S&P classifications except for bonds purchased for loss mitigation or risk management strategies, which use internal ratings classifications. The Company’s portfolio consists primarily of high-quality, liquid instruments.

[3] Accumulated OCI ("AOCI"). See also Note 17, Shareholders' Equity.

[4] Government-agency obligations were approximately 45% of mortgage backed securities as of March 31, 2014 and 50% as of December 31, 2013 based on fair value.