Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	$ 14,409	$ 13,791
Fair value of asset	27,002	25,701
Separate accounts	948	925
Top Hat savings plan mutual funds and common equity (included in Other assets)	77	67
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	7,725	7,452
States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	4,611	4,703
Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	1,607	1,224
Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	247	187
United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	186	200
Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	33	25
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	186	200
Fair value of asset	11,546	10,908
Top Hat savings plan mutual funds and common equity (included in Other assets)	77	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	186	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	14,223	13,591
Fair value of asset	15,456	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	7,725	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	4,611	4,703
Level 2 | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	1,607	1,224
Level 2 | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	247	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	33	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	11,283	10,641
Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	351	348
Equity securities | Level 1 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair value of asset	11,283	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	351	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	882	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	882	854
Separate accounts taxable fixed maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	$ 0	$ 0