Business Combination - Summary of Operating Results of ETFS since the Acquisition Date (Detail) - USD ($) $ in Thousands	3 Months Ended							9 Months Ended	12 Months Ended
	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	[1]	Dec. 31, 2018	[1]
Business Combinations [Abstract]
Revenues								$ 55,882
Income before taxes								23,197
(Loss)/gain on revaluation of deferred consideration	$ (5,354)	$ (6,306)	$ (4,037)	$ 4,404	$ (5,410)	$ 7,732	$ 9,898	$ 12,220	$ (11,293)		$ 12,220

[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