Derivative instruments - Additional Information (Detail) In Millions, unless otherwise specified			0 Months Ended	12 Months Ended
	Sep. 30, 2010 Cross-currency swap [Member] Designated as Hedging Instrument [Member] Pay Euros [Member] EUR (€)	Sep. 30, 2010 Cross-currency swap [Member] Designated as Hedging Instrument [Member] Receive U.S. dollars [Member] USD ($)	Jun. 30, 2008 Interest Rate Swap [Member] De-designated hedges [Member] USD ($)	Dec. 31, 2010 Interest Rate Swap [Member] De-designated hedges [Member] USD ($)
Derivatives, Fair Value [Line Items]
Notional amount	€ 28.7	$ 39.0	$ 150.0
Fixed rate (as a percent)	5.00%	4.635%
Gain on settlement of derivative instrument				$ 1.3
Period of amortizable interest rate swap agreement			3 years