Fair Value Measurements (Details) - Fair Value Hierarchy - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Level within the fair value hierarchy at which the Company's financial assets and financial liabilities are measured
Estimated fair value of put/call option reported in other liabilities	$ 10
Recurring basis [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	63,884	$ 63,204
Other liabilities measured on a recurring basis	10
Recurring basis [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	63,464	62,694
Recurring basis [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	2,064	2,076
Recurring basis [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	28,611	30,915
Recurring basis [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	1,257	1,509
Recurring basis [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	2,573	2,410
Recurring basis [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	28,880	25,689
Recurring basis [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	79	95
Recurring basis [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	368	453
Recurring basis [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	316	339
Recurring basis [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	52	114
Recurring basis [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	52	57
Recurring basis [Member] | 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	2,429	2,493
Recurring basis [Member] | 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	2,067	2,090
Recurring basis [Member] | 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	2,064	2,076
Recurring basis [Member] | 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
Recurring basis [Member] | 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
Recurring basis [Member] | 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	346	384
Recurring basis [Member] | 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	316	339
Recurring basis [Member] | 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	30	45
Recurring basis [Member] | 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	16	19
Recurring basis [Member] | Level 1 [Member] | Investments in various publicly-traded securities, including mutual funds and other small holdings [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	16	19
Recurring basis [Member] | 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	61,233	60,469
Recurring basis [Member] | 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	61,211	60,400
Recurring basis [Member] | 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	28,599	30,910
Recurring basis [Member] | 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	1,257	1,509
Recurring basis [Member] | 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	2,554	2,371
Recurring basis [Member] | 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	28,725	25,518
Recurring basis [Member] | 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	76	92
Recurring basis [Member] | 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	22	69
Recurring basis [Member] | 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	22	69
Recurring basis [Member] | 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	222	242
Other liabilities measured on a recurring basis	10
Estimated fair value of put/call option reported in other liabilities	10
Recurring basis [Member] | 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	186	204
Recurring basis [Member] | 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
Recurring basis [Member] | 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	19	39
Recurring basis [Member] | 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	155	160
Recurring basis [Member] | 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	36	38
Recurring basis [Member] | Level 3 [Member] | Investment in non-public common and preferred equity securities where the fair value estimate is determined either internally or by an external fund manager [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	$ 36	$ 38