Fair Value Hierarchy for Assets Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities debt Securities	$ 8,779	$ 8,383
Available for sale securities equity securities	2,956	3,041
Separate accounts	671	621
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of asset	12,371	12,041
Fair Value, Measurements, Recurring | Top Hat Savings Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	8	9
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	8,779	8,383
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	3,252	3,143
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	59	69
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	7	5
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	160	200
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	5,298	4,963
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	2,854	2,940
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	102	101
Fair Value, Measurements, Recurring | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts	628	608
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	2,869	2,954
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	8	9
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	7	5
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	7	5
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	2,854	2,940
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	9,477	9,056
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	8,751	8,354
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	3,249	3,139
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	59	69
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	160	200
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	5,280	4,943
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	98	96
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	628	606
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	31
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	21	24
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	3	4
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	18	20
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	4	5
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3 | Taxable Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Separate accounts		$ 2