Investments of Operating Entities and Consolidated Funds - Indirect Concentration of the Underlying Investments Held by Consolidated Funds (Details) $ in Thousands	Mar. 31, 2018 USD ($) investments	Dec. 31, 2017 USD ($) investments
Investment [Line Items]
Concentration of risk, number of investments | investments	1	1
Equity Securities [Member] | Linkem [Member] | ITALY | 517210 Wireless Telecommunications Carriers (except Satellite) [Member] (Deprecated 2016-08-31)
Investment [Line Items]
Percentage of Equity	7.77%	7.52%
Market Value | $	$ 58,947	$ 56,271