Derivative Instruments - Additional Information (Detail) (Foreign Exchange Forward) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended						9 Months Ended
	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Sep. 30, 2012 EUR (€)	Sep. 30, 2012 GBP (£)	Sep. 30, 2011 EUR (€)	Sep. 30, 2011 GBP (£)	Sep. 30, 2012 Maximum D
Derivative [Line Items]
Term of derivative contracts (in days)									90
Notional amount of forward foreign currency sale contracts					€ 48.9	£ 39.0	€ 22.5	£ 16.5
Cash received on sale of derivative contracts			123.6	58.6
Net gain (loss) on forward foreign currency contracts as a result of change in fair value	$ 0.9	$ 0.5	$ 1.5	$ 0.3