Segment Reporting - Reportable Segment Information (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	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Operating Revenues:
Total revenues	$ 68,907	$ 67,718	$ 66,293	$ 65,485	$ 67,867	$ 72,570	$ 74,775	$ 58,904		$ 268,403		$ 274,116		$ 228,295
Total operating expenses										(214,869)		(212,837)		(179,207)
Other income/(expenses)
Interest expense										(11,240)		(7,962)
Interest income										3,332		3,093		2,861
(Loss)/gain on revaluation of deferred consideration – gold payments	(5,354)	(6,306)	(4,037)	4,404	(5,410)	7,732	9,898		$ 12,220	(11,293)	[1]	12,220	[1]
Impairment	30,138	0	0	572	17,386					30,710		17,386
Settlement gain														6,909
Other gains/(losses), net										(3,502)		(205)		(666)
Income/(loss) before taxes	$ (22,355)	$ 8,635	$ 6,066	$ 7,775	$ (12,597)	$ 27,530	$ 22,184	$ 13,922		121		51,039		58,192
Advisory Fees [Member]
Operating Revenues:
Total revenues										265,652		271,104		226,692
U.S. Business segment [Member]
Operating Revenues:
Other income										339		608		520
Total revenues										170,828		204,906		217,541
Total operating expenses										(141,067)		(152,430)		(156,342)
Other income/(expenses)
Interest expense										(780)		(566)
Interest income										3,326		3,093		2,861
Impairment										(30,710)		(17,386)
Settlement gain														6,909
Other gains/(losses), net										272		292		(432)
Other income/(expenses)										(27,892)		(14,567)		9,338
Income/(loss) before taxes										1,869		37,909		70,537
U.S. Business segment [Member] | Advisory Fees [Member]
Operating Revenues:
Total revenues										170,489		204,298		217,021
International Business Segment [Member]
Operating Revenues:
Other income										2,412		2,404		1,083
Total revenues										97,575		69,210		10,754
Total operating expenses										(73,802)		(60,407)		(22,865)
Other income/(expenses)
Interest expense										(10,460)		(7,396)
Interest income										6
(Loss)/gain on revaluation of deferred consideration – gold payments										(11,293)		12,220
Other gains/(losses), net										(3,774)		(497)		(234)
Other income/(expenses)										(25,521)		4,327		(234)
Income/(loss) before taxes										(1,748)		13,130		(12,345)
International Business Segment [Member] | Advisory Fees [Member]
Operating Revenues:
Total revenues										$ 95,163		$ 66,806		$ 9,671

[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