Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	$ 9,522	$ 9,121
Equity securities, at fair value	4,666	4,375
Separate accounts	730	692
Fair value of asset	14,899	14,192
Multiemployer Plan, Plan Information, Not Available [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	17	14
Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	9,522	9,121
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	3,464	3,211
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	17
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
Debt Securities | Government-Sponsored Enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	210	200
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 | Commercial Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	254	143
Debt Securities | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5,570	5,533
Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	4,488	4,213
Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	178	162
Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	694	682
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	4,512	4,234
Quoted prices in active markets for identical assets (Level 1) | Multiemployer Plan, Plan Information, Not Available [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	17	14
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 | 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 | 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) | 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 | Commercial Mortgage Backed Securities
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 | Corporate Securities
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 Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	4,488	4,213
Quoted prices in active markets for identical assets (Level 1) | Equity Securities | Preferred Stock
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	10,357	9,954
Significant other observable inputs (Level 2) | Multiemployer Plan, Plan Information, Not Available [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	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	9,487	9,112
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	3,464	3,211
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	17
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 | Government-Sponsored Enterprises
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	210	200
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 | Commercial Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	254	143
Significant other observable inputs (Level 2) | Debt Securities | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	5,542	5,531
Significant other observable inputs (Level 2) | Equity Securities | Common Stock
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 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	176	160
Significant other observable inputs (Level 2) | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	694	682
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	30	4
Significant unobservable inputs (Level 3) | Multiemployer Plan, Plan Information, Not Available [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	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	28	2
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	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) | 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 | Commercial Mortgage Backed Securities
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 | Corporate Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	28	2
Significant unobservable inputs (Level 3) | Equity Securities | Common Stock
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 | Preferred Stock
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