Investments of Operating Entities and Consolidated Funds - Merger Master Derivatives (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative Asset		$ 32,229	$ 69,177
Derivative Liability		30,002	42,750
Merger Master
Derivative [Line Items]
Derivative Asset		3,675	122,551
Derivative Liability		2,266	2,718
Option
Derivative [Line Items]
Derivative Liability	[1]	27,184	41,563
Option | Merger Master
Derivative [Line Items]
Derivative Liability		261	1,256
Options
Derivative [Line Items]
Derivative Asset	[2]	29,330	58,172
Options | Merger Master
Derivative [Line Items]
Derivative Asset		1,817	108,063
Swaps
Derivative [Line Items]
Derivative Asset		1,203	10,942
Derivative Liability		2,743	71
Swaps | Merger Master
Derivative [Line Items]
Derivative Asset		1,658	14,488
Derivative Liability		2,005	1,249
Currency Forwards
Derivative [Line Items]
Derivative Asset		1,088	5
Derivative Liability		4	874
Currency Forwards | Merger Master
Derivative [Line Items]
Derivative Asset		200	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.