Investments of Operating Entities and Consolidated Funds - Securities borrowed and securities loaned (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Securities Borrowed, Gross		$ 713,288	$ 443,148
Securities borrowed, gross amounts offset on the condensed consolidated statements of financial condition	[1]	0	0
Securities borrowed, net amounts included on the condensed consolidated statements of financial condition		713,288	443,148
Securities borrowed, additional amounts, asset		0	0
Securities borrowed, financial instruments		684,569	414,778
Securities borrowed, cash collateral pledged	[2]	0	0
Securities borrowed, net amounts		28,719	28,370
Securities Loaned, Gross		548,629	456,831
Securities Loaned, Gross amounts offset on the Condensed Consolidated Statements of Financial Condition	[1]	0	0
Securities Loaned, Gross amounts recognized		548,629	456,831
Securities Loaned, Additional Amounts Available		0	0
Securities Loaned, Financial instruments		531,984	439,228
Securities Loaned, Cash Collateral pledged	[2]	0	0
Securities Loaned, Net amounts		$ 16,645	$ 17,603

[1] Includes financial instruments subject to enforceable master netting provisions that are permitted to be offset to the extent an event of default has occurred.

[2] Includes the amount of cash collateral held/posted.