Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions | Jun. 30, 2015 | Dec. 31, 2014 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | $ 9,675 | $ 9,460 |
Equity securities, at fair value | 4,734 | 4,858 |
Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0) | 75 | 0 |
Separate accounts | 742 | 752 |
Fair value of asset | 15,236 | 15,067 |
Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0) | 75 | |
Top Hat savings plan mutual funds and common equity (included in Other assets) | | |
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) | 20 | 18 |
Debt Securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | 9,675 | 9,460 |
Debt Securities | Corporate | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | 5,577 | 5,526 |
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 | 3,547 | 3,443 |
Debt Securities | Commercial mortgage-backed | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | 283 | 259 |
Debt Securities | Government-sponsored enterprises | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | 244 | 208 |
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 | 7 | 7 |
Debt Securities | United States government | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | 7 | 7 |
Equity Securities | Common equities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 4,502 | 4,679 |
Equity Securities | Nonredeemable preferred equities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Equity securities, at fair value | 232 | 179 |
Taxable Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Separate accounts | 732 | 731 |
Quoted prices in active markets for identical assets (Level 1) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fair value of asset | 4,529 | 4,704 |
Quoted prices in active markets for identical assets (Level 1) | Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0) | 0 | |
Quoted prices in active markets for identical assets (Level 1) | Top Hat savings plan mutual funds and common equity (included in Other assets) | | |
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) | 20 | 18 |
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 | 7 | 7 |
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 | 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) | Debt Securities | United States government | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fixed maturities, at fair value | 7 | 7 |
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 | 4,502 | 4,679 |
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] | | |
Fair value of asset | 10,686 | 10,343 |
Significant other observable inputs (Level 2) | Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0) | 75 | |
Significant other observable inputs (Level 2) | Top Hat savings plan mutual funds and common equity (included in Other assets) | | |
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 |
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 | 9,649 | 9,435 |
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 | 5,559 | 5,508 |
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 | 3,546 | 3,443 |
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 | 283 | 259 |
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 | 244 | 208 |
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 | 7 | 7 |
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) | 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 | 230 | 177 |
Significant other observable inputs (Level 2) | Taxable Fixed Maturities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Separate accounts | 732 | 731 |
Significant unobservable inputs (Level 3) | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Fair value of asset | 21 | 20 |
Significant unobservable inputs (Level 3) | Short-term investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0) | 0 | |
Significant unobservable inputs (Level 3) | Top Hat savings plan mutual funds and common equity (included in Other assets) | | |
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 |
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 | 19 | 18 |
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 | 18 | 18 |
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 | 1 | 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 | 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) | 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) | 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 | 2 | 2 |
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 |