Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	$ 10,540	$ 10,085
Equity securities, at fair value	6,025	5,334
Separate accounts	798	766
Top Hat savings plan mutual funds and common equity (included in Other assets)	30	24
Fair value of asset	17,378	16,193
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	10,540	10,085
Debt Securities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5,783	5,781
Debt Securities | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	4,234	3,828
Debt Securities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	288	287
Debt Securities | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	204	164
Debt Securities | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	16	10
Debt Securities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	10	10
Debt Securities | Convertibles and bonds with warrants attached
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5	5
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	5,808	5,123
Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	217	211
Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	783	750
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Top Hat savings plan mutual funds and common equity (included in Other assets)	30	24
Fair value of asset	5,854	5,157
Quoted prices in active markets for identical assets (Level 1) | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	16	10
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	16	10
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Convertibles and bonds with warrants attached
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	5,808	5,123
Quoted prices in active markets for identical assets (Level 1) | Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Fair value of asset	11,518	10,958
Significant other observable inputs (Level 2) | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	10,518	9,997
Significant other observable inputs (Level 2) | Debt Securities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5,782	5,703
Significant other observable inputs (Level 2) | Debt Securities | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	4,229	3,828
Significant other observable inputs (Level 2) | Debt Securities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	288	287
Significant other observable inputs (Level 2) | Debt Securities | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	204	164
Significant other observable inputs (Level 2) | Debt Securities | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Significant other observable inputs (Level 2) | Debt Securities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	10	10
Significant other observable inputs (Level 2) | Debt Securities | Convertibles and bonds with warrants attached
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5	5
Significant other observable inputs (Level 2) | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	0	0
Significant other observable inputs (Level 2) | Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	217	211
Significant other observable inputs (Level 2) | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	783	750
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Top Hat savings plan mutual funds and common equity (included in Other assets)	0	0
Fair value of asset	6	78
Significant unobservable inputs (Level 3) | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	6	78
Significant unobservable inputs (Level 3) | Debt Securities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	1	78
Significant unobservable inputs (Level 3) | Debt Securities | States, municipalities and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5	0
Significant unobservable inputs (Level 3) | Debt Securities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Significant unobservable inputs (Level 3) | Debt Securities | Government-sponsored enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Significant unobservable inputs (Level 3) | Debt Securities | United States government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Significant unobservable inputs (Level 3) | Debt Securities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Significant unobservable inputs (Level 3) | Debt Securities | Convertibles and bonds with warrants attached
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	0	0
Significant unobservable inputs (Level 3) | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	0	0
Significant unobservable inputs (Level 3) | Equity securities | Nonredeemable preferred equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	0	0
Significant unobservable inputs (Level 3) | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	$ 0	$ 0