Investments - Summary of Investments (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Equity Investments, at Fair Value		$ 20,189	$ 34,220
Total Trading Securities		8,938	5,283
Investments in Equity Method Investees		9,906	10,967
Total	[1]	39,033	50,470
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Equity Investments, at Fair Value		13,377	9,567
Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Equity Investments, at Fair Value		6,812	24,653
U.S. Treasury Bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Trading Securities		$ 8,938	$ 5,283

[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.