Fair Value Hierarchy for Assets Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt Securities	$ 8,956	$ 8,779
Available for sale securities equity securities	3,131	2,956
Separate accounts	686	671
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	12,730	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	8,956	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	3,259	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	56	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	103	160
Fair Value, Measurements, Recurring | Debt Securities | Foreign Government
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	5,527	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	3,014	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	117	102
Fair Value, Measurements, Recurring | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	634	628
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	3,031	2,869
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | 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 | Fair Value, Inputs, Level 1 | 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 | Fair Value, Inputs, Level 1 | 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 | Fair Value, Inputs, Level 1 | Equity Securities | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	3,014	2,854
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	9,674	9,477
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt Securities	8,930	8,751
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | 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	3,257	3,249
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | 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	56	59
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | 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	103	160
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Debt Securities | Foreign Government
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 | Fair Value, Inputs, Level 2 | Debt Securities | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt Securities	5,511	5,280
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Equity Securities | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities equity securities	110	98
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	634	628
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	25	25
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt Securities	18	21
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | 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	2	3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Debt Securities | Corporate Bond Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt Securities	16	18
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | 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