Financial instruments - Gain (Losses) on Derivative and Non-Derivative Financial Instruments (Details) - Designated as Hedging Instrument - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Cross-currency swap contracts
Derivative Instruments, Gain (Loss)
Derivative and non derivative instruments gain (loss) recognized in accumulated other comprehensive income (loss)	$ (2,725)	$ 8,907	$ (9,298)	$ 5,019
Foreign denominated debt
Derivative Instruments, Gain (Loss)
Derivative and non derivative instruments gain (loss) recognized in accumulated other comprehensive income (loss)	$ (10,893)	$ 4,934	$ (27,727)	$ 4,261