Fair Value Measurements (details) - Fair Value Hierarchy (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	$ 64,654	$ 63,438
Fixed maturities [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	64,000	62,820
Fixed maturities [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,234	2,025
Fixed maturities [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	61,534	60,565
Fixed maturities [Member] | Level 3
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	232	230
U.S. Treasury securities and obligations of U.S. Government and government agencies and authorities [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,231	2,008
U.S. Treasury securities and obligations of U.S. Government and government agencies and authorities [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,198	1,991
U.S. Treasury securities and obligations of U.S. Government and government agencies and authorities [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	33	17
Obligations of states, municipalities and political subdivisions [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	39,275	39,535
Obligations of states, municipalities and political subdivisions [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	39,254	39,433
Obligations of states, municipalities and political subdivisions [Member] | Level 3
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	21	102
Debt securities issued by foreign governments [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,002	2,202
Debt securities issued by foreign governments [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,002	2,202
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	3,618	4,164
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	3,617	4,163
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 3
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	1	1
All other corporate bonds [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	16,837	14,876
All other corporate bonds [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	16,627	14,749
All other corporate bonds [Member] | Level 3
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	210	127
Redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	37	35
Redeemable preferred stock [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	36	34
Redeemable preferred stock [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	1	1
Equity securities [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	570	519
Equity securities [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	498	412
Equity securities [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	72	107
Equity securities, common stock [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	376	304
Equity securities, common stock [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	364	281
Equity securities, common stock [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	12	23
Equity securities, non-redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	194	215
Equity securities, non-redeemable preferred stock [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	134	131
Equity securities, non-redeemable preferred stock [Member] | Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	60	84
Other investments [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	84	99
Other investments [Member] | Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	39	42
Other investments [Member] | Level 3
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	45	57
Level 1
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,771	2,479
Level 2
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	61,606	60,672
Level 3
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	$ 277	$ 287