Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	3 Months Ended	6 Months Ended	113 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unrealized Loss / Gain on Auction Rate Securities Tender Offer	$ 2,400,000
Auction Rate Securities Purchased as a Result of Settlements with Regulators			$ 137,200,000
Securities owned, at fair value	995,370,000	$ 995,370,000	995,370,000	$ 837,584,000
Forward or delayed delivery of the underlying instrument with settlement		180 days
Total amount of ARS the firm purchased and hold		$ 48,500,000
Auction Rate Securities Committed To Purchase related to settlements with regulators	2,000,000	2,000,000	2,000,000
Auction Rate Securities Purchased as a result of Legal Settlements and Awards			102,300,000
Auction Rate Securities Committed To Purchase as a Result of Legal Settlements and Awards	3,600,000	3,600,000	3,600,000
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	39,430,000	39,430,000	39,430,000	28,215,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 42,461,000	$ 42,461,000	$ 42,461,000	$ 37,952,000
Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities Committed To Purchase Period		2020
Hedge Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment Redemption Notice Period Minimum		30 days
Investment Redemption Notice Period Maximum		120 days
Private Equity Funds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments Lock In Period		10 years