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Fair Value Measurements - Additional Information (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 561,683	$ 610,517
Forward or delayed delivery of the underlying instrument with settlement	180 days
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	$ 561,683	610,517
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	77,643	94,750
Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	48,744	36,554
Auction rate securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	31,804	30,701
Auction rate securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities owned, at fair value	$ 0	$ 0
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

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Maximum settlement period in case of delayed delivery of underlying instrument.

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Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned

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Fair value of financial instruments held by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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