Fair Value Measurement- Quantitative Information about Level 3 Fair Value Measurements of Assets (Detail) (USD $) In Millions, unless otherwise specified				3 Months Ended			3 Months Ended			3 Months Ended				3 Months Ended						3 Months Ended				3 Months Ended
	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3	Dec. 31, 2012 Fair Value, Measurements, Recurring Level 3	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Available-for-sale State and political subdivisions Discounted Cash Flow	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Available-for-sale State and political subdivisions Discounted Cash Flow Minimum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Available-for-sale State and political subdivisions Discounted Cash Flow Maximum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Distressed Debt Discounted Cash Flow	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Distressed Debt Discounted Cash Flow Minimum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Distressed Debt Discounted Cash Flow Maximum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Structured Foreign Exchange Swaptions Option Pricing Model	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Structured Foreign Exchange Swaptions Option Pricing Model Minimum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Structured Foreign Exchange Swaptions Option Pricing Model Maximum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Long-Term Foreign Exchange Options Option Pricing Model	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Equity Options Option Pricing Model	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Equity Options Option Pricing Model Minimum	Mar. 31, 2013 Fair Value, Measurements, Recurring Level 3 Trading Assets Equity Options Option Pricing Model Maximum	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring Level 3	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3 Loans	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3 Loans Minimum	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3 Loans Maximum	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3 Loans Discounted Cash Flow	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3 Loans Discounted Cash Flow Minimum	Mar. 31, 2013 Fair Value, Measurements, Nonrecurring Level 3 Loans Discounted Cash Flow Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs Expected Credit Loss				6.00%	37.00%
Fair value of assets	$ 253	$ 315	$ 44			$ 11			$ 10			$ 1	$ 31			$ 251	$ 285	$ 21	$ 23				$ 21
Correlation risk (percent)										0.00%	25.00%
Volatility (percent)										11.00%	16.00%	18.00%		23.00%	28.00%
Expected maturity							1 year	15 years
Credit spreads (percent)							2.00%	9.50%
Timing of sale																					0 months	12 months
Cap rate (percent)																				8.00%
Cost to complete/sell (percent)																								0.00%	30.00%