Financial instruments - Financial assets and liabilities measured at fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019
Assets:
Equity securities	$ 7,133	$ 7,533
Derivatives	53,380	38,244
Liabilities:
Derivatives	6,282	4,512
Measured at Net Asset Value
Assets:
Investments measured at net asset value	2,612	9,728
Quoted Prices In Active Markets (Level 1)
Assets:
Equity securities	7,133	7,533
Derivatives	0	0
Liabilities:
Derivatives	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Equity securities	0	0
Derivatives	53,380	38,244
Liabilities:
Derivatives	6,282	4,512
Significant Unobservable Inputs (Level 3)
Assets:
Equity securities	0	0
Derivatives	0	0
Liabilities:
Derivatives	$ 0	$ 0