Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020
ASSETS
Deposits with clearing organizations	$ 25,118	$ 23,991
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	799,719
Securities owned
Securities owned, at fair value	799,719	610,517
Investments	5,816	4,836
Derivative contracts
Total	824,837	638,704
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	100,571	126,171
Investments		0
Derivative contracts:
Derivative contracts	1,414	220
Total	101,985	126,391
Investments
Derivative contracts:
Transfers In (Out)	(106)
Equity Securities [Member]
Securities owned
Securities owned, at fair value	32,215	36,554
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		17,892
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	781	200
Auction rate securities
Securities owned
Securities owned, at fair value	25,314	30,701
Corporate equities
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	21,978
U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	613,030
Securities owned
Securities owned, at fair value		448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	52,882	93,261
US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	35,891	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18
US Government Agencies Debt Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		9
Sovereign obligations
Securities owned
Securities owned, at fair value	11,405	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,405	623
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	8,310	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	664	5,283
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,697	3,103
Municipal obligations
Securities owned
Securities owned, at fair value	40,260	25,190
Convertible bonds
Securities owned
Securities owned, at fair value	29,816	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,624	9,103
Other Investments
Securities owned
Securities owned, at fair value		4,181
Level 1
ASSETS
Deposits with clearing organizations	25,118	23,991
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	665,943
Securities owned
Securities owned, at fair value		485,066
Derivative contracts
Total	691,061	509,057
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	74,860	111,153
Investments		0
Derivative contracts:
Derivative contracts	267	22
Total	75,127	111,175
Level 1 | Equity Securities [Member]
Securities owned
Securities owned, at fair value	32,215	36,554
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		17,892
Level 1 | Money Market Funds [Member]
Securities owned
Securities owned, at fair value	781	200
Level 1 | Auction rate securities
Securities owned
Securities owned, at fair value		0
Level 1 | Corporate equities
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	21,978
Level 1 | U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	613,030
Securities owned
Securities owned, at fair value		448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	52,882	93,261
Level 1 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	19,917	0
Level 2
ASSETS
Deposits with clearing organizations		0
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	133,776
Securities owned
Securities owned, at fair value		94,750
Derivative contracts
Total	133,776	98,946
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	25,711	15,018
Derivative contracts:
Derivative contracts	1,147	3
Total	26,858	15,021
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	25,314	0
Level 2 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	15,974	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18	9
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	11,405	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,405	623
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	8,310	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	664	5,283
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,697	3,103
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	40,260	25,190
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	29,816	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,624	9,103
Level 2 | Other Investments
Securities owned
Securities owned, at fair value		4,181
Level 3
ASSETS
Deposits with clearing organizations		0
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	0
Securities owned
Securities owned, at fair value		30,701
Derivative contracts
Total	0	30,701
Derivative contracts:
Derivative contracts	0	195
Total	0	195
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	0	30,701
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	0	0
TBAs
Derivative contracts
Derivative contracts, total		15
Derivative contracts:
Derivative contracts	124	3
TBAs | Level 2
Derivative contracts
Derivative contracts, total		15
Derivative contracts:
Derivative contracts	124	3
ARS purchase commitments
Derivative contracts:
Derivative contracts	1,023	195
ARS purchase commitments | Level 2
Derivative contracts:
Derivative contracts	1,023	0
ARS purchase commitments | Level 3
Derivative contracts:
Derivative contracts	0	195
Future [Member]
Derivative contracts:
Derivative contracts	267	22
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	$ 267	$ 22