Investments - Summary of Investments (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Equity Investments, at Fair Value		$ 29,010	$ 5,452
Total Trading Securities		12,951	0
Investments in Equity Method Investees		16,209	16,285
Total	[1]	58,170	21,737
Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Equity Investments, at Fair Value		6,962	5,452
Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Equity Investments, at Fair Value		22,048	0
U.S. Treasury Bills
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Trading Securities		11,662	0
Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Trading Securities		$ 1,289	$ 0

[1] Asset and liability amounts in parentheses represent the aggregated balances at September 30, 2018 and December 31, 2017 attributable to Pzena International Value Service (a series of Pzena Investment Management, LLC), Pzena Investment Management Special Situations, LLC, and Pzena U.S. Best Ideas (GP), LLC, which were variable interest entities as of September 30, 2018 and December 31, 2017, respectively. Aggregated balances of variable interest entities at September 30, 2018 also reflect Pzena Global Best Ideas (GP), LLC.