Significant Accounting Policies - Summary of Reclassifications to Conform to the Current Year's Consolidated Financial Statement Presentation (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating Revenues:
Advisory fees		$ 55,856		$ 108,884
Settlement gain		6,909		6,909
Other income		386		725
Total revenues (currently reported)	$ 74,775	56,242	$ 133,679	109,609
Other Income/(Expenses):
Other gains and losses, net	$ (501)	(364)	$ (762)	284
Previously Reported [Member]
Operating Revenues:
Advisory fees		56,114		109,376
Settlement gain		6,909		6,909
Other income		405		1,742
Restatement Adjustment [Member] | Advisory Fees Reclassified to Other Income [Member]
Operating Revenues:
Advisory fees		(258)		(492)
Restatement Adjustment [Member] | Settlements Gain Reclassified to Other Income Expenses [Member]
Operating Revenues:
Settlement gain		(6,909)		(6,909)
Restatement Adjustment [Member] | Other Exchange Trade Product Fees Reclassified From Advisory Fees [Member]
Operating Revenues:
Other income		258		492
Restatement Adjustment [Member] | Interest Income Reclassified to Other Income Expenses [Member]
Operating Revenues:
Other income		(641)		(1,225)
Restatement Adjustment [Member] | Realized and Unrealized Losses Reclassified to Other Gains and Losses Net [Member]
Operating Revenues:
Other income		364		519
Restatement Adjustment [Member] | Miscellaneous Other Income Reclassified To Other Gains And Losses, Net [Member]
Operating Revenues:
Other income				(803)
Restatement Adjustment [Member] | Interest Income Reclassified From Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net		641		1,225
Restatement Adjustment [Member] | Settlement Gain Reclassified from Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net		6,909		6,909
Restatement Adjustment [Member] | Realized and Unrealized Losses Reclassified From Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net		$ (364)		(519)
Restatement Adjustment [Member] | Miscellaneous Other Income Reclassified From Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net				$ 803