Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($)	9 Months Ended	116 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction Rate Securities Purchased as a Result of Settlements with Regulators		$ 140,200,000
Securities owned, at fair value	$ 653,194,000	653,194,000	$ 837,584,000
Forward or delayed delivery of the underlying instrument with settlement	180 days
Total amount of ARS the firm purchased and hold	$ 29,400,000	29,400,000
Auction Rate Securities Committed To Purchase related to settlements with regulators	3,400,000	3,400,000
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
ARS Securities Committed To Purchase Period	2020
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 31,822,000	31,822,000	28,215,000
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 25,314,000	$ 25,314,000	$ 37,952,000
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