Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	$ 11,600	$ 10,689
Equity securities, at fair value	7,176	5,920
Separate accounts	878	803
Top Hat savings plan mutual funds and common equity (included in Other assets)	45	34
Fair value of asset	19,689	17,434
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)	45	34
Fair value of asset	7,110	5,843
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	12,579	11,586
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	5
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	11,600	10,689
Debt Securities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	6,410	5,710
Debt Securities | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	4,590	4,304
Debt Securities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	305	288
Debt Securities | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	162	310
Debt Securities | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	109	67
Debt Securities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	24	10
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	109	67
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) | Government-sponsored enterprises
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	109	67
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 | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	11,491	10,617
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	6,410	5,709
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	4,590	4,300
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	305	288
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	162	310
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	24	10
Debt Securities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fixed maturities, at fair value	0	5
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	1
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	4
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) | Government-sponsored enterprises
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
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	6,956	5,742
Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Equity securities, at fair value	220	178
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	6,956	5,742
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	220	178
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	868	791
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	868	791
Taxable Fixed Maturities | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Separate accounts	$ 0	$ 0