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ACCOUNTS PAYABLE AND ACCRUED LIABILITIES - Narrative (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Jun. 30, 2019
Accounts Payable and Accrued Liabilities [Abstract]    
Present value of asset retirement obligation $ 17.3 $ 16.0
Undiscounted value of asset retirement obligation $ 18.7 $ 17.6