Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	$ 14,084	$ 13,791
Fair value of asset	26,585	25,701
Separate accounts	927	925
Top Hat savings plan mutual funds and common equity (included in Other assets)	78	67
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	7,599	7,452
States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	4,668	4,703
Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	1,421	1,224
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	189	187
United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	178	200
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	29	25
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	178	200
Fair value of asset	11,459	10,908
Top Hat savings plan mutual funds and common equity (included in Other assets)	78	67
Level 1 | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 1 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 1 | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 1 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 1 | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	178	200
Level 1 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	13,906	13,591
Fair value of asset	15,126	14,793
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Level 2 | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	7,599	7,452
Level 2 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	4,668	4,703
Level 2 | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	1,421	1,224
Level 2 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	189	187
Level 2 | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 2 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	29	25
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Fair value of asset	0	0
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Level 3 | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 3 | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 3 | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 3 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 3 | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Level 3 | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	11,203	10,641
Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	354	348
Equity securities | Level 1 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	11,203	10,641
Equity securities | Level 1 | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	0	0
Equity securities | Level 2 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	0	0
Equity securities | Level 2 | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	354	348
Equity securities | Level 3 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	0	0
Equity securities | Level 3 | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	0	0
Separate accounts taxable fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	866	854
Separate accounts taxable fixed maturities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	0	0
Separate accounts taxable fixed maturities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	866	854
Separate accounts taxable fixed maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	$ 0	$ 0