Investments of Operating Entities and Consolidated Funds - Merger Master Derivatives (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Asset		$ 69,177	$ 22,901
Derivative Liability		42,750	20,762
Merger Master [Member]
Derivative [Line Items]
Derivative Asset		122,551	4,519
Derivative Liability		2,718	2,408
Put Option
Derivative [Line Items]
Derivative Liability	[1]	41,563	19,939
Put Option | Merger Master [Member]
Derivative [Line Items]
Derivative Liability		1,256	2,285
Options
Derivative [Line Items]
Derivative Asset	[2]	58,172	21,539
Options | Merger Master [Member]
Derivative [Line Items]
Derivative Asset		108,063	4,264
Swaps
Derivative [Line Items]
Derivative Asset		10,942	468
Derivative Liability		71	181
Swaps | Merger Master [Member]
Derivative [Line Items]
Derivative Asset		14,488	255
Derivative Liability		1,249	123
Currency Forwards
Derivative [Line Items]
Derivative Asset		5	592
Derivative Liability		874	0
Currency Forwards | Merger Master [Member]
Derivative [Line Items]
Derivative Liability		$ 213	$ 0

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

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