Deferred Consideration - Additional Information (Detail)		3 Months Ended	6 Months Ended	9 Months Ended
	Apr. 11, 2018 USD ($)	Sep. 30, 2018 USD ($) oz	Sep. 30, 2018 USD ($) oz	Sep. 30, 2018 USD ($) oz
Business Acquisition [Line Items]
Fixed payments ounces of gold per year | oz		9,500	9,500	9,500
Upfront consideration paid				$ 0
Deferred consideration	$ 172,746,000	$ 156,055,000	$ 156,055,000	156,055,000
Deferred consideration, current		11,788,000	11,788,000	11,788,000
Deferred consideration, non-current		$ 144,267,000	144,267,000	$ 144,267,000
Discount rate used to value the deferred consideration	10.00%	10.00%		10.00%
Contractual gold payments		$ 2,880,000		$ 5,595,000
Gain/loss on revaluation of deferred consideration		$ 7,732,000	$ 17,630,000	$ 17,630,000
April 1, 2018 through March 31, 2058 [Member]
Business Acquisition [Line Items]
Fixed payments ounces of gold per year | oz		9,500	9,500	9,500
April 1, 2058 and Thereafter [Member]
Business Acquisition [Line Items]
Fixed payments ounces of gold per year | oz		6,333	6,333	6,333