Significant Accounting Policies - Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Sep. 30, 2019		Jun. 30, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents
Cash	$ 35,685	[1]	$ 33,783	$ 38,099	[1]	$ 31,913	$ 52,541	$ 63,414
Restricted Cash	1,033		1,031	1,028		1,026	1,024	1,017
Total	$ 36,718		$ 34,814	$ 39,127		$ 32,939	$ 53,565	$ 64,431

[1] Asset and liability amounts in parentheses represent the aggregated balances at September 30, 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 September 30, 2019 and December 31, 2018, respectively.