Business Combination - Additional Information (Detail) - USD ($)		3 Months Ended							9 Months Ended	12 Months Ended
	Apr. 11, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
(Loss)/gain on revaluation of deferred consideration		$ (5,354,000)	$ (6,306,000)	$ (4,037,000)	$ 4,404,000	$ (5,410,000)	$ 7,732,000	$ 9,898,000	$ 12,220,000	$ (11,293,000)	[1]	$ 12,220,000	[1]
Acquisition and disposition-related costs										902,000		11,454,000		$ 4,832,000
Acquisition of ETFS Business [Member]
(Loss)/gain on revaluation of deferred consideration												$ (740,000)		$ (2,380,000)
Business acquisition, cash paid	$ 253,000,000
Business acquisition, number of shares to be issued	30,000,000
Business acquisition, number of common shares that the preferred stock is convertible into	14,750,000
Acquisition and disposition-related costs										$ 608,000
Acquisition of ETFS Business [Member] | Term Loan Facility [Member]
Borrowing under term loan facility	$ 200,000,000
Acquisition of ETFS Business [Member] | Revolving Credit Facility [Member]
Revolving credit facility maximum borrowing capacity	$ 50,000,000
Acquisition of ETFS Business [Member] | Common Stock [Member]
Business acquisition, number of shares to be issued	15,250,000
Series A Non-Voting Convertible Preferred Stock [Member] | Acquisition of ETFS Business [Member]
Business acquisition, number of shares to be issued	14,750

[1] (Losses)/gains arise due to increases/(decreases) in the forward-looking price of gold and the magnitude of any gain or loss is highly correlated to the magnitude of the change in the forward-looking price of gold. See Note 5 for a reconciliation of changes in the deferred consideration balances