Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction Rate Securities Purchased as a Result of Settlements with Regulators	$ 140,200
Securities owned, at fair value	$ 309,346	$ 799,719
Forward or delayed delivery of the underlying instrument with settlement	180 days
Total amount of ARS the firm purchased and hold	$ 26,300
Auction Rate Securities Committed To Purchase related to settlements with regulators	2,300
Auction Rate Securities Purchased as a result of Legal Settlements and Awards	102,300
Auction Rate Securities Committed To Purchase as a Result of Legal Settlements and Awards	$ 3,700
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	$ 25,582	32,215
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 26,336	$ 25,314
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