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Derivative instruments - Additional Information (Detail) (Cross-currency swap [Member])
12 Months Ended
Dec. 31, 2006
Terminated hedges [Member]
USD ($)
Sep. 30, 2010
Terminated hedges [Member]
USD ($)
Sep. 30, 2010
Pay Euros [Member]
Designated as Hedging Instrument [Member]
EUR (€)
Sep. 30, 2010
Receive U.S. dollars [Member]
Designated as Hedging Instrument [Member]
USD ($)
Derivatives, Fair Value [Line Items]        
Amortization term of instrument (in years) 10 years      
Notional amount $ 63,000,000   € 33,500,000 $ 45,500,000
Cash settlement amount   $ 450,000    
Fixed rate (as a percent)     5.00% 4.635%