Supplemental Financial Information - Quarterly Results (Detail) - USD ($) $ / shares in Units, $ 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	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 68,907	$ 67,718	$ 66,293	$ 65,485	$ 67,867	$ 72,570	$ 74,775	$ 58,904		$ 268,403		$ 274,116		$ 228,295
Operating income	14,809	16,131	11,911	10,683	11,819	21,708	14,531	13,221		53,534		61,279		49,088
(Loss)/income before income taxes	(22,355)	8,635	6,066	7,775	(12,597)	27,530	22,184	13,922		121		51,039		58,192
Net (loss)/income	$ (25,880)	$ 4,152	$ 2,479	$ 8,824	$ (11,564)	$ 22,049	$ 16,724	$ 9,424		$ (10,425)		$ 36,633		$ 27,199
(Loss)/earnings per share - basic	$ (0.17)	$ 0.02	$ 0.01	$ 0.05	$ (0.08)	$ 0.13	$ 0.10	$ 0.07		$ (0.08)		$ 0.23		$ 0.20
(Loss)/earnings per share - diluted	(0.17)	0.02	0.01	0.05	(0.08)	0.13	0.10	0.07		(0.08)		0.23		0.20
Dividends per common share	$ 0.03	$ 0.03	$ 0.03	$ 0.03	$ 0.03	$ 0.03	$ 0.03	$ 0.03		$ 0.12		$ 0.12		$ 0.32
(Loss)/gain on revaluation of deferred consideration (Note 12)	$ (5,354)	$ (6,306)	$ (4,037)	$ 4,404	$ (5,410)	$ 7,732	$ 9,898		$ 12,220	$ (11,293)	[1]	$ 12,220	[1]
Impairments (Note 26)	$ (30,138)	$ 0	$ 0	$ (572)	$ (17,386)					$ (30,710)		$ (17,386)

[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