Fair Value Measurements (Details) - Fair Value Hierarchy - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
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	$ 5	$ 8
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	74,508	68,595
Other liabilities measured on a recurring basis	5	8
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	74,003	68,134
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	2,149	2,095
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	36,360	29,917
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	1,054	1,173
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	2,361	3,280
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	32,054	31,619
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	25	50
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	453	425
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	410	383
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	43	42
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	36
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	2,597	2,507
Other liabilities measured on a recurring basis	0	0
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	2,152	2,095
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	2,149	2,095
Recurring basis [Member] | Level 1 [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	0	0
Recurring basis [Member] | Level 1 [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	0	0
Recurring basis [Member] | Level 1 [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	0	0
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	0	0
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	0
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	428	396
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	410	383
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	18	13
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	17	16
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	17	16
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	71,710	65,967
Other liabilities measured on a recurring basis	0	0
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	71,685	65,938
Recurring basis [Member] | Level 2 [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	0	0
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	36,349	29,905
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	1,054	1,173
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	2,361	3,280
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	31,899	31,530
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	22	50
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	25	29
Recurring basis [Member] | 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	0	0
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	25	29
Recurring basis [Member] | Level 2 [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	0	0
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	201	121
Other liabilities measured on a recurring basis	5	8
Estimated fair value of put/call option reported in other liabilities	5	8
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	166	101
Recurring basis [Member] | Level 3 [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	0	0
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	11	12
Recurring basis [Member] | Level 3 [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	0	0
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	0	0
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	89
Recurring basis [Member] | Level 3 [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	0	0
Recurring basis [Member] | Level 3 [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	0	0
Recurring basis [Member] | Level 3 [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	0	0
Recurring basis [Member] | Level 3 [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	0	0
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	35	20
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	$ 35	$ 20