Derivative Instruments - Effects of Trading Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Trading Activity, Gains and Losses, Net [Line Items]
Unrealized gain (loss) on forward contracts Revenue	$ 81	$ 84	$ (137)	$ 183
Total	407	997		745	2,086
Revenue [Member] | Forward Contracts [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Unrealized gain (loss) on forward contracts Revenue	86	86		239	(147)
Revenue [Member] | Forward Contracts [Member] | Options [Member]
Trading Activity, Gains and Losses, Net [Line Items]
Realized gain on forward contracts and options	$ 321	$ 911		$ 506	$ 2,233