Derivative Instruments - Effects of Trading Activities on Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Trading Activity, Gains and Losses, Net [Line Items]
Unrealized gain (loss) on forward contracts Revenue	$ 186	$ (352)	$ 18
Total	1,344	2,356	2,256
Revenue | Forward Contracts
Trading Activity, Gains and Losses, Net [Line Items]
Unrealized gain (loss) on forward contracts Revenue	217	(339)	41
Revenue | Forward Contracts | Put Option
Trading Activity, Gains and Losses, Net [Line Items]
Realized gain on forward contracts/put option Revenue	$ 1,127	$ 2,695	$ 2,215