Fair Value Measurements (Details) - Fair Value Hierarchy - Recurring basis [Member] - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	$ 62,916	$ 63,204
Other liabilities measured on a recurring basis	19
Fixed maturities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	62,424	62,694
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 and financial liabilities are measured
Total invested assets measured on a recurring basis	2,017	2,076
Obligations of states, municipalities and political subdivisions [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	28,181	30,915
Debt securities issued by foreign governments [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	1,279	1,509
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	2,491	2,410
All other corporate bonds [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	28,367	25,689
Redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	89	95
Equity securities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	426	453
Public common stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	347	339
Non-redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	79	114
Other Investments [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	66	57
Level 1 [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	2,418	2,493
Level 1 [Member] | Fixed maturities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	2,020	2,090
Level 1 [Member] | 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 and financial liabilities are measured
Total invested assets measured on a recurring basis	2,017	2,076
Level 1 [Member] | All other corporate bonds [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis		11
Level 1 [Member] | Redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	3	3
Level 1 [Member] | Equity securities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	380	384
Level 1 [Member] | Public common stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	346	339
Level 1 [Member] | Non-redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	34	45
Level 1 [Member] | Other Investments [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	18	19
Level 2 [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	60,256	60,469
Level 2 [Member] | Fixed maturities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	60,210	60,400
Level 2 [Member] | Obligations of states, municipalities and political subdivisions [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	28,169	30,910
Level 2 [Member] | Debt securities issued by foreign governments [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	1,279	1,509
Level 2 [Member] | Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	2,471	2,371
Level 2 [Member] | All other corporate bonds [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	28,205	25,518
Level 2 [Member] | Redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	86	92
Level 2 [Member] | Equity securities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	46	69
Level 2 [Member] | Public common stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	1
Level 2 [Member] | Non-redeemable preferred stock [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	45	69
Level 3 [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	242	242
Other liabilities measured on a recurring basis	19
Level 3 [Member] | Fixed maturities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	194	204
Level 3 [Member] | Obligations of states, municipalities and political subdivisions [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	12	5
Level 3 [Member] | Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	20	39
Level 3 [Member] | All other corporate bonds [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	162	160
Level 3 [Member] | Other Investments [Member]
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Total invested assets measured on a recurring basis	$ 48	$ 38