Investments - Summary of Other Equity Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	May 31, 2017	Dec. 31, 2016	Nov. 30, 2016
Atalanta Sosnoff [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment				$ 14,730
Other Equity Method Investments [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	$ 59,856		$ 79,717
Other Equity Method Investments [Member] | G5 [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	0	$ 11,555	26,016
Other Equity Method Investments [Member] | ABS [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	39,894		38,982
Other Equity Method Investments [Member] | Atalanta Sosnoff [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	13,963		14,719
Other Equity Method Investments [Member] | Luminis [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	$ 5,999		$ 0