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Financial instruments - Notional Amounts and Fair Values of Derivatives by Product (Detail) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Derivatives, Fair Value [Line Items]    
Derivatives asset, Notional   $ 100,000
Derivatives asset, Fair Value   20
Not Designated as Hedging Instrument [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative liability, notional 6,179,343 6,062,292
Derivative liability, Fair Value 3,057 2,421
Not Designated as Hedging Instrument [Member] | Euro Dollars Futures [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative liability, notional 15,000  
Derivative liability, Fair Value 4  
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member] | U.S. Treasury Futures [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative liability, notional 56,000 20,000
Derivative liability, Fair Value 286 66
Not Designated as Hedging Instrument [Member] | Commodity Contract [Member] | Federal Funds Futures [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative liability, notional 6,070,000 5,985,000
Derivative liability, Fair Value 120 8
Not Designated as Hedging Instrument [Member] | Other Contract [Member] | Auction rate securities purchase commitment [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative liability, notional 38,343 57,292
Derivative liability, Fair Value 2,647 2,347
Not Designated as Hedging Instrument [Member] | Interest Rate Contract [Member] | Interest Rate Cap [Member]
    
Derivatives, Fair Value [Line Items]    
Derivatives asset, Notional   100,000
Derivatives asset, Fair Value   $ 20