Investments of Operating Entities and Consolidated Funds - Caerus Master Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Trading Securities	$ 673,221	$ 700,876
Securities sold, not yet purchased, at fair value	342,527	266,090
Common Stock
Investment [Line Items]
Securities sold, not yet purchased, at fair value	$ 342,328	263,460
Caerus Master [Member] \| Common Stock
Investment [Line Items]
Trading Securities		3,200
Securities sold, not yet purchased, at fair value		$ 2,600

Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

Name:	us-gaap_TradingSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant