Deferred Consideration - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($) oz	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Fixed payment ounces of gold | oz	2,375
Upfront consideration paid	$ 0
Deferred consideration	157,147,000	$ 161,540,000
Deferred consideration, current	11,857,000	11,765,000
Deferred consideration, non-current	$ 145,290,000	$ 149,775,000
Discount rate used to value the deferred consideration	10.00%
Contractual gold payments	$ 3,098,000
Gain/loss on revaluation of deferred consideration	$ 4,404,000
April 1, 2018 through March 31, 2058 [Member]
Business Acquisition [Line Items]
Fixed payment ounces of gold | oz	9,500
April 1, 2058 and Thereafter [Member]
Business Acquisition [Line Items]
Fixed payment ounces of gold | oz	6,333