Financial instruments - Notional Amounts and Fair Values of Derivatives by Product (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative liability, notional	$ 7,426,901	$ 6,179,343
Derivative liability, Fair Value	2,525	3,057
Euro Dollars Futures [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, notional		15,000
Derivative liability, Fair Value		4
Commodity Contract [Member] | U.S. Treasury Futures [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, notional	57,600	56,000
Derivative liability, Fair Value	360	286
Commodity Contract [Member] | Federal Funds Futures [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, notional	7,330,000	6,070,000
Derivative liability, Fair Value	71	120
Other Contract [Member] | Auction rate securities purchase commitments [Member]
Derivatives, Fair Value [Line Items]
Derivative liability, notional	39,301	38,343
Derivative liability, Fair Value	$ 2,094	$ 2,647