Investments - Gains and Losses Recognized in Gain (Loss) on Securities, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
(Loss) Gain on Securities [Line Items]
Gain (Loss) on Securities, net	$ (586)	$ 261	$ 1,670	$ (1,736)
Investments—Consolidated Investment Companies
(Loss) Gain on Securities [Line Items]
Unrealized Gains (Losses)	(364)	606	1,345	(1,737)
Realized Gains	79	99	239	1,424
Realized Losses	(198)	(623)	(1,025)	(1,521)
Gain (Loss) on Securities, net	(483)	82	559	(1,834)
Investments—Affiliates and Other
(Loss) Gain on Securities [Line Items]
Unrealized Gains (Losses)	2,298	142	3,515	(64)
Net Realized Gains (Losses) Recognized on Securities Sold	(2,401)	37	(2,404)	162
Gain (Loss) on Securities, net	$ (103)	$ 179	$ 1,111	$ 98