Fair Value Measurements (details) - Fair Value Hierarchy - Recurring basis [Member] - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Level within the fair value hierarchy at which the Company's financial assets are measured
Total invested assets measured on a recurring basis	$ 63,829	$ 61,419
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	2,761	2,813
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	60,817	58,350
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	251	256
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	63,036	60,658
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	2,077	2,197
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	60,740	58,243
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	219	218
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	2,071	2,194
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	2,071	2,194
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	33,280	31,411
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	33,268	31,398
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	12	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	1,757	1,873
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	1,757	1,873
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	1,778	1,981
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	1,743	1,957
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	35	24
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	24,047	23,089
All other corporate bonds [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
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	23,872	22,915
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	172	174
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	103	110
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	3
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	100	100
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
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	744	705
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	667	598
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	77	107
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	609	543
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	609	543
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	135	162
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	58	55
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	77	107
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	49	56
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	17	18
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	$ 32	$ 38