Fair Value Measurements (details) - Fair Value Hierarchy - Recurring basis [Member] - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	$ 62,819	$ 64,428
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	2,779	2,843
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	59,784	61,317
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	256	268
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	61,933	63,474
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	2,053	2,051
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	59,666	61,191
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	214	232
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	2,046	2,053
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	2,043	2,049
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	3	4
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	32,710	33,573
Obligations of states, municipalities and political subdivisions [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	7
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	32,689	33,560
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	14	13
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	2,100	2,368
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	2,100	2,368
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	2,046	2,213
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	2,010	2,203
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	36	10
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	22,908	23,135
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	22,751	22,934
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	157	201
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	123	132
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	3	2
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	113	122
Redeemable preferred stock [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	7	8
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	828	899
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	708	773
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	118	126
Equity 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	2
Public 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	639	691
Public 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	637	691
Public common stock [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	2
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	189	208
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	71	82
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	118	126
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	58	55
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	18	19
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	$ 40	$ 36