Significant Accounting Policies - Additional Information (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) segment		Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)		Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Right-of-use asset	$ 14,704,000			$ 0
Lease liability	$ 15,066,000			0
Revenue Recognition
Contract measurement period (in years)	3 years
Cash and Cash Equivalents
Cash	$ 14,731,000	[1]	$ 27,347,000	38,099,000	[1]	$ 63,414,000
Restricted Cash	1,029,000		1,021,000	1,028,000		$ 1,017,000
Impairment losses recognized	0		0
Concentration of Credit Risk
Allowance for doubtful accounts receivable	$ 0			0
Lease term	12 months
Business Segments
Number of operating segments (in segments) | segment	1
Tax Receivable Agreement:
Valuation allowance on deferred tax assets	$ 0			$ 0
Section 754 election tax deduction period	15 years
Cash savings generated by this election will be distributed to selling and converting shareholders upon realization (in hundredth)	85.00%
Percentage of cash tax savings in income tax if realizes company expects to benefit	15.00%
Cash savings generated by this election will increase additional paid-in capital	15.00%
Other comprehensive income associated with foreign currency translation adjustments	$ 63,000		$ 68,000
Equipment | Minimum
Concentration of Credit Risk
Property and equipment, useful life (years)	3 years
Equipment | Maximum
Concentration of Credit Risk
Property and equipment, useful life (years)	7 years
Largest Customer | Customer Concentration Risk | Advisory fees
Concentration of Credit Risk
Concentration risk (percent)	9.50%		12.20%
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Right-of-use asset	$ 11,700,000
Lease liability	$ 11,900,000

[1] Asset and liability amounts in parentheses represent the aggregated balances at March 31, 2019 and December 31, 2018 attributable to Pzena International Value Service (a series of Pzena Investment Management, LLC), Pzena Investment Management Special Situations, LLC, Pzena U.S. Best Ideas (GP), LLC, and Pzena Global Best Ideas (GP), LLC which were variable interest entities as of March 31, 2019 and December 31, 2018, respectively.