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Accounts Payable and Accrued Liabilities - Narrative (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Jun. 30, 2017
Accounts Payable and Accrued Liabilities [Abstract]    
Present value of asset retirement obligation $ 15.7 $ 13.4
Undiscounted value of asset retirement obligation $ 17.7 $ 15.0