Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 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	$ 142,500
Securities owned, at fair value	$ 646,680	$ 799,719
Forward or delayed delivery of the underlying instrument with settlement	180 days
Auction Rate Securities Purchased as a result of Legal Settlements and Awards	$ 106,100
Auction Rate Securities remaining to purchase from Eligible Investors related to the settlements with the Regulators	1,300
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	646,680	799,719
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	32,121	32,215
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 30,701	$ 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