Investments of Operating Entities and Consolidated Funds - Merger Master Derivatives (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Asset		$ 34,913	$ 69,177
Derivative Liability		48,875	42,750
Merger Master [Member]
Derivative [Line Items]
Derivative Asset		70,682	122,551
Derivative Liability		2,096	2,718
Put Option
Derivative [Line Items]
Derivative Liability	[1]	47,423	41,563
Put Option | Merger Master [Member]
Derivative [Line Items]
Derivative Liability		1,845	1,256
Options
Derivative [Line Items]
Derivative Asset	[2]	31,216	58,172
Options | Merger Master [Member]
Derivative [Line Items]
Derivative Asset		69,017	108,063
Swaps
Derivative [Line Items]
Derivative Asset		1,631	10,942
Derivative Liability		623	71
Swaps | Merger Master [Member]
Derivative [Line Items]
Derivative Asset		1,665	14,488
Derivative Liability		251	1,249
Currency Forwards
Derivative [Line Items]
Derivative Asset		520	5
Derivative Liability		620	874
Currency Forwards | Merger Master [Member]
Derivative [Line Items]
Derivative Asset		0	0
Derivative Liability		$ 0	$ 213

[1] Includes index, equity, commodity future and cash conversion options.

[2] Includes index, equity, commodity future and cash conversion options.