Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 hedge_fund	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets	$ 3,181	$ 3,884	$ 3,690	$ 4,157
Derivative, Number of Instruments Held	30
Transfers Into Level 3	(47)	(3)
Long-term debt from Level 3 to Level 2	86	174
Deferred fee on stand by letters of credit	45		46
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Interest in Hedge Finds, Percentage Discounted	60.00%
Derivative Instrument, Foreign Currency, Implied Volatility Percentage	20.90%
Derivative Instrument, Equity, Implied Volatility	120.00%
Derivative Instruments, Foreign Currency, Correlations of a Group, Percentage	61.00%
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Interest in Hedge Finds, Percentage Discounted	50.00%
Derivative Instrument, Foreign Currency, Implied Volatility Percentage	1.60%
Derivative Instrument, Equity, Implied Volatility	5.00%
Derivative Instruments, Foreign Currency, Correlations of a Group, Percentage	52.00%
Loss Severity Rates [Member] | Residential Mortgage [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Discount Rate	22.00%
Loss Severity Rates [Member] | Residential Mortgage [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Inputs, Discount Rate	59.00%
Derivative Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets	1,300	2,000
Derivative Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value assets	$ 782	$ 2,200