Derivative Instruments - Forward Contracts Outstanding, Estimated Fair Value, Change in Fair Value and Realized Gains of Derivative Instruments (Detail) - Forward Contracts £ in Thousands, AUD in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2014 AUD	Dec. 31, 2013 GBP (£)	Dec. 31, 2013 AUD
Derivative [Line Items]
Fair Value	$ 0	$ 149
Net Foreign Exchange (Losses) Gains	$ 0	$ 1,475
Australian dollar
Derivative [Line Items]
Contract Date	Nov. 26, 2013	Nov. 26, 2013
Settlement Date	Jan. 03, 2014	Jan. 03, 2014
Contract Amount and Settlement Amount	$ 41,036	$ 41,036	AUD 45,000		AUD 45,000
Fair Value	0	779
Net Foreign Exchange (Losses) Gains	$ (130)	$ 779
U.S Dollar
Derivative [Line Items]
Contract Date	Jul. 01, 2013	Jul. 01, 2013
Settlement Date	Jan. 03, 2014	Jan. 03, 2014
Contract Amount and Settlement Amount	$ 40,887	$ 40,887	AUD 45,000		45,000
Fair Value	0	(630)
Net Foreign Exchange (Losses) Gains	$ 130	$ (630)
Australian dollar 2
Derivative [Line Items]
Contract Date		Feb. 08, 2012
Settlement Date		May 10, 2013
Contract Amount and Settlement Amount		$ 36,099			AUD 45,000
Fair Value		0
Net Foreign Exchange (Losses) Gains		$ 303
British pound
Derivative [Line Items]
Contract Date		Mar. 06, 2012
Settlement Date		Mar. 06, 2013
Contract Amount and Settlement Amount		$ 26,611		£ 17,000
Fair Value		0
Net Foreign Exchange (Losses) Gains		$ 1,023