Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	$ 9,116	$ 8,779
Available for sale securities equity securities	3,349	2,956
Separate accounts	715	671
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	13,150	12,371
Fair Value, Measurements, Recurring | Top Hat Savings Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	9	8
Fair Value, Measurements, Recurring | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	9,116	8,779
Fair Value, Measurements, Recurring | Debt Securities | US States and Political Subdivisions Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	3,288	3,252
Fair Value, Measurements, Recurring | Debt Securities | Convertibles and Bonds with Warrants Attached
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	31	59
Fair Value, Measurements, Recurring | Debt Securities | US Government Agencies Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	8	7
Fair Value, Measurements, Recurring | Debt Securities | US Government-sponsored Enterprises Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	116	160
Fair Value, Measurements, Recurring | Debt Securities | Foreign Government Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	3	3
Fair Value, Measurements, Recurring | Debt Securities | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	5,670	5,298
Fair Value, Measurements, Recurring | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	3,210	2,854
Fair Value, Measurements, Recurring | Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	139	102
Fair Value, Measurements, Recurring | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	676	628
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	3,227	2,869
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Top Hat Savings Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	9	8
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	8	7
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Debt Securities | US Government Agencies Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	8	7
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	3,210	2,854
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	9,911	9,477
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	9,103	8,751
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Debt Securities | US States and Political Subdivisions Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	3,287	3,249
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Debt Securities | Convertibles and Bonds with Warrants Attached
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	31	59
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Debt Securities | US Government-sponsored Enterprises Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	116	160
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Debt Securities | Foreign Government Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	3	3
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Debt Securities | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	5,666	5,280
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	132	98
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	676	628
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	12	25
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	5	21
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Debt Securities | US States and Political Subdivisions Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	1	3
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Debt Securities | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt securities	4	18
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	$ 7	$ 4