Fair value measurement - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Transfers Net		$ 0
Financial instruments measured at fair value on a recurring basis, transfers out of Level 3	(8)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	2
Cost To Sell	5	1
Fair Value, Level 3 Transfers out, Description	The transfers from Level 3 to Level 2 of trading mortgage-backed securities resulted from observable market data becoming available for these securities.
Fair Value, Level 3 Transfers in, Description	The transfers from Level 2 to Level 3 of trading mortgage-backed securities were the result of a change in valuation technique to a matrix pricing model, based on indicative prices provided by brokers.
Fair Value Level 2 To Level 1 Transfers Amount	0	0
Fair Value Level 1 To Level 2 Transfers Amount	$ 0	$ 0
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Contingent Liability [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs Discount Rate Percentage	6.60%
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Contingent Liability [Member] | Fair Value, Measurements, Recurring [Member] | First Methodology [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs Discount Rate Percentage	6.60%
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Contingent Liability [Member] | Fair Value, Measurements, Recurring [Member] | Second Methodology [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value Inputs Discount Rate Percentage	6.70%