Fair Value Measurements (Details) - Fair Value Hierarchy - Recurring basis [Member] - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
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 $ 68,272 $ 68,595
Other liabilities measured on a recurring basis 7 8
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 67,897 68,134
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 2,055 2,095
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 31,008 29,917
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 1,040 1,173
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 3,307 3,280
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 30,441 31,619
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 46 50
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 342 425
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 305 383
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 37 42
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 33 36
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 2,392 2,507
Other liabilities measured on a recurring basis 0 0
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 2,060 2,095
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 2,055 2,095
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
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
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
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 2 0
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
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 320 396
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 305 383
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 15 13
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 12 16
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 65,765 65,967
Other liabilities measured on a recurring basis 0 0
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 65,743 65,938
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
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 30,996 29,905
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 1,040 1,173
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 3,307 3,280
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 30,357 31,530
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 43 50
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 29
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
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 29
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
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 115 121
Other liabilities measured on a recurring basis 7 8
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 94 101
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
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 12
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
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
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 82 89
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
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
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
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
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 $ 21 $ 20