Financial instruments - Financial Assets & Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jun. 30, 2017
Derivatives
Liabilities:
Derivative liability	$ 15,495	$ 16,064
Quoted Prices In Active Markets (Level 1) | Derivatives
Liabilities:
Derivative liability	0	0
Significant Other Observable Inputs (Level 2) | Derivatives
Liabilities:
Derivative liability	15,495	16,064
Significant Unobservable Inputs (Level 3) | Derivatives
Liabilities:
Derivative liability	0	0
Equity securities
Assets:
Investments	3,682	3,008
Equity securities | Quoted Prices In Active Markets (Level 1)
Assets:
Investments	3,682	3,008
Equity securities | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Equity securities | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0	0
Corporate bonds
Assets:
Investments	6,026	5,968
Corporate bonds | Quoted Prices In Active Markets (Level 1)
Assets:
Investments	6,026	5,968
Corporate bonds | Significant Other Observable Inputs (Level 2)
Assets:
Investments	0	0
Corporate bonds | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0	0
Asset-backed and mortgage-backed securities
Assets:
Investments	4,369	4,762
Asset-backed and mortgage-backed securities | Quoted Prices In Active Markets (Level 1)
Assets:
Investments	0	0
Asset-backed and mortgage-backed securities | Significant Other Observable Inputs (Level 2)
Assets:
Investments	4,369	4,762
Asset-backed and mortgage-backed securities | Significant Unobservable Inputs (Level 3)
Assets:
Investments	0	0
Derivatives
Assets:
Derivative asset	2,098	16,496
Derivatives | Quoted Prices In Active Markets (Level 1)
Assets:
Derivative asset	0	0
Derivatives | Significant Other Observable Inputs (Level 2)
Assets:
Derivative asset	2,098	16,496
Derivatives | Significant Unobservable Inputs (Level 3)
Assets:
Derivative asset	0	0
Investments measured at net asset value
Assets:
Investments	$ 7,614	$ 7,073