Investments - Gains and Losses Recognized in Gain (Loss) on Securities, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
(Loss) Gain on Securities [Line Items]
Gain (Loss) on Securities, net	$ 261	$ 1,667	$ (1,736)	$ 6,463
Investments—Consolidated Investment Companies
(Loss) Gain on Securities [Line Items]
Unrealized Gains (Losses)	606	(437)	(1,737)	659
Realized Gains	99	1,110	1,424	3,184
Realized Losses	(623)	(66)	(1,521)	(765)
Gain (Loss) on Securities, net	82	607	(1,834)	3,078
Investments—Affiliates and Other
(Loss) Gain on Securities [Line Items]
Unrealized Gains (Losses)	142	89	(64)	360
Net Realized Gains Recognized on Securities Sold	37	971	162	3,025
Gain (Loss) on Securities, net	$ 179	$ 1,060	$ 98	$ 3,385