Investments - Summary of Other Equity Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	$ 60,241	$ 59,856
ABS [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	40,440	39,894
Atalanta Sosnoff [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	13,802	13,963
Luminis [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment	$ 5,999	$ 5,999