Significant Accounting Policies - Summary of Reclassifications to Conform to the Current Year's Consolidated Financial Statement Presentation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Revenues:
Advisory fees		$ 57,293		$ 166,177
Settlement gain				6,909
Other income		421		1,145
Total revenues (currently reported)	$ 72,570	57,714	$ 206,249	167,322
Other Income/(Expenses):
Other gains and losses, net	$ 118	(500)	$ (644)	(217)
Previously Reported [Member]
Operating Revenues:
Advisory fees		57,574		166,950
Settlement gain				6,909
Other income		412		2,154
Restatement Adjustment [Member] | Advisory Fees Reclassified to Other Income [Member]
Operating Revenues:
Advisory fees		(281)		(773)
Restatement Adjustment [Member] | Settlements Gain Reclassified to Other Income Expenses [Member]
Operating Revenues:
Settlement gain				(6,909)
Restatement Adjustment [Member] | Other Exchange Trade Product Fees Reclassified From Advisory Fees [Member]
Operating Revenues:
Other income		281		773
Restatement Adjustment [Member] | Interest Income Reclassified to Other Income Expenses [Member]
Operating Revenues:
Other income		(772)		(1,999)
Restatement Adjustment [Member] | Realized and Unrealized Losses Reclassified to Other Gains and Losses Net [Member]
Operating Revenues:
Other income		535		1,054
Restatement Adjustment [Member] | Miscellaneous Other Income Reclassified To Other Gains And Losses, Net [Member]
Operating Revenues:
Other income		(35)		(837)
Restatement Adjustment [Member] | Interest Income Reclassified From Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net		772		1,999
Restatement Adjustment [Member] | Settlement Gain Reclassified from Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net				6,909
Restatement Adjustment [Member] | Realized and Unrealized Losses Reclassified From Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net		(535)		(1,054)
Restatement Adjustment [Member] | Miscellaneous Other Income Reclassified From Operating Revenues [Member]
Other Income/(Expenses):
Other gains and losses, net		$ 35		$ 837