Fair Value Measurements (details) - Fair Value Hierarchy (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	$ 65,472	$ 66,138
Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,838	2,885
Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	62,331	62,969
Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	303	284
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	64,699	65,393
Fixed maturities [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,151	2,237
Fixed maturities [Member] | Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	62,294	62,926
Fixed maturities [Member] | Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	254	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	2,136	2,222
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,119	2,205
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member] | Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	17	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	38,246	38,681
Obligations of states, municipalities and political subdivisions [Member] | Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	38,219	38,653
Obligations of states, municipalities and political subdivisions [Member] | Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	27	28
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	2,189	2,257
Debt securities issued by foreign governments [Member] | Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,189	2,257
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	2,776	2,997
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	2,761	2,992
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member] | Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	15	5
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	19,319	19,203
All other corporate bonds [Member] | Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	19,107	19,006
All other corporate bonds [Member] | Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	212	197
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	33	33
Redeemable preferred stock [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	32	32
Redeemable preferred stock [Member] | Level 2 [Member]
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	694	645
Equity securities [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	657	602
Equity securities [Member] | Level 2 [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	43
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	561	510
Equity securities, common stock [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	561	510
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	133	135
Equity securities, non-redeemable preferred stock [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	96	92
Equity securities, non-redeemable preferred stock [Member] | Level 2 [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	43
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	79	100
Other investments [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	30	46
Other investments [Member] | Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	49	54
Investments in various publicly-traded securities, including securities in the Company's trading portfolio, mutual funds and other small holdings disclosed in Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	30	46
Investments in various publicly-traded securities, including securities in the Company's trading portfolio, mutual funds and other small holdings disclosed in Level 1 [Member] | Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	30	46
Investment in non-public common and preferred equity securities where the fair value estimate is determined either internally or by an external fund manager and therefore disclosed in Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	49	54
Investment in non-public common and preferred equity securities where the fair value estimate is determined either internally or by an external fund manager and therefore disclosed in Level 3 [Member] | Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	$ 49	$ 54