Investments - Gains and Losses Recognized in (Loss) Gain on Securities, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Securities [Line Items]
(Loss) Gain on Securities, net	$ (4,357)	$ 8,072	$ 2,108
Investments—Consolidated Investment Companies
Gain (Loss) on Securities [Line Items]
Unrealized (Losses) Gains	(3,142)	771	4,232
Realized Gains	1,596	3,318	2,358
Realized Losses	(1,970)	(1,073)	(3,678)
(Loss) Gain on Securities, net	(3,516)	3,016	2,912
Investments—Affiliates and Other
Gain (Loss) on Securities [Line Items]
Unrealized (Losses) Gains	(1,180)	118	392
Net Realized Gains (Losses) Recognized on Securities Sold	339	4,938	(1,196)
(Loss) Gain on Securities, net	$ (841)	$ 5,056	$ (804)