Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	$ 12,157	$ 11,698
Equity securities, at fair value	7,867	7,752
Separate accounts	929	882
Top Hat savings plan mutual funds and common equity (included in Other assets)	46	45
Fair value of asset	20,979	20,350
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	46	45
Fair value of asset	7,759	7,667
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Fair value of asset	13,220	12,683
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Fair value of asset	0	0
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	12,157	11,698
Debt Securities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	6,769	6,401
Debt Securities | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	4,942	4,728
Debt Securities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	282	301
Debt Securities | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	124	104
Debt Securities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	27	28
Debt Securities | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	13	136
Debt Securities | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	124	104
Debt Securities | Quoted prices in active markets for identical assets (Level 1) | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Quoted prices in active markets for identical assets (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
Debt Securities | Quoted prices in active markets for identical assets (Level 1) | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Quoted prices in active markets for identical assets (Level 1) | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	124	104
Debt Securities | Quoted prices in active markets for identical assets (Level 1) | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Quoted prices in active markets for identical assets (Level 1) | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	12,033	11,594
Debt Securities | Significant other observable inputs (Level 2) | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	6,769	6,401
Debt Securities | Significant other observable inputs (Level 2) | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	4,942	4,728
Debt Securities | Significant other observable inputs (Level 2) | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	282	301
Debt Securities | Significant other observable inputs (Level 2) | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant other observable inputs (Level 2) | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	27	28
Debt Securities | Significant other observable inputs (Level 2) | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	13	136
Debt Securities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant unobservable inputs (Level 3) | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant unobservable inputs (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
Debt Securities | Significant unobservable inputs (Level 3) | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant unobservable inputs (Level 3) | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant unobservable inputs (Level 3) | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	0
Debt Securities | Significant unobservable inputs (Level 3) | Government-sponsored enterprises
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
Equity securities, at fair value	7,589	7,518
Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	278	234
Equity securities | Quoted prices in active markets for identical assets (Level 1) | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	7,589	7,518
Equity securities | Quoted prices in active markets for identical assets (Level 1) | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	0	0
Equity securities | Significant other observable inputs (Level 2) | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	0	0
Equity securities | Significant other observable inputs (Level 2) | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	278	234
Equity securities | Significant unobservable inputs (Level 3) | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	0	0
Equity securities | Significant unobservable inputs (Level 3) | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	0	0
Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	909	855
Taxable Fixed Maturities | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	0	0
Taxable Fixed Maturities | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	909	855
Taxable Fixed Maturities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	$ 0	$ 0