Derivatives & Risk Management - Summary of Location of Gains and Losses on the Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Aug. 31, 2015	Aug. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Realized (gain) loss on derivatives	$ 141	$ (2,177)
Foreign currency forward contracts \| Realized gain on derivatives \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized (gain) loss on derivatives	141	(2,177)
Foreign currency forward contracts \| Unrealized (gain) loss on derivatives \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized (gain) loss on derivatives	(2,047)	1,501
Variable cash settlement \| Share-based payments expense \| Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Realized (gain) loss on derivatives	$ 111	$ (169)