Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
ASSETS
Deposits with clearing organizations	$ 27,899	$ 23,991
Securities owned
Securities owned, at fair value	561,683	610,517
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	561,683	610,517
Investments	4,771	4,181
Investments	5,570	4,836
Derivative contracts
Derivative contracts, total	526
Total	594,879	638,704
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	84,145	126,171
Derivative contracts:
Derivative contracts	531	220
Total	84,676	126,391
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts		195
Equity Securities [Member]
Securities owned
Securities owned, at fair value	48,744	36,554
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	16,232	17,892
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	794	200
Auction rate securities
Securities owned
Securities owned, at fair value	31,804	30,701
U.S. Treasury securities
Securities owned
Securities owned, at fair value	402,698	448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	43,972	93,261
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	7,849	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4	9
Sovereign obligations
Securities owned
Securities owned, at fair value	3,657	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		623
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	17,983	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,286	5,283
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,609	3,103
Municipal obligations
Securities owned
Securities owned, at fair value	28,393	25,190
Convertible bonds
Securities owned
Securities owned, at fair value	17,152	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,647
Mortgage Backed Securities, Other
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4
Level 1
ASSETS
Deposits with clearing organizations	27,899	23,991
Securities owned
Securities owned, at fair value	452,236	485,066
Derivative contracts
Derivative contracts, total	0
Total	480,135	509,057
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	60,204	111,153
Derivative contracts:
Derivative contracts	11	22
Total	60,215	111,175
Level 1 \| Corporate equities
Securities owned
Securities owned, at fair value	48,744	36,554
Level 1 \| Money markets
Securities owned
Securities owned, at fair value	794	200
Level 1 \| Auction rate securities
Securities owned
Securities owned, at fair value	0
Level 1 \| U.S. Treasury securities
Securities owned
Securities owned, at fair value	402,698	448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	43,972	93,261
Level 1 \| US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	0	0
Level 1 \| Corporate debt and other obligations
Securities owned
Securities owned, at fair value	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Level 1 \| Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	16,232	17,892
Level 2
Securities owned
Securities owned, at fair value	77,643	94,750
Investments	4,771	4,181
Derivative contracts
Derivative contracts, total	526
Total	82,940	98,946
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	23,941	15,018
Derivative contracts:
Derivative contracts	520	3
Total	24,461	15,021
Level 2 \| Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts		0
Level 2 \| Auction rate securities
Securities owned
Securities owned, at fair value	0	0
Level 2 \| US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	7,849	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4	9
Level 2 \| Sovereign obligations
Securities owned
Securities owned, at fair value	3,657	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		623
Level 2 \| Corporate debt and other obligations
Securities owned
Securities owned, at fair value	17,983	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,286	5,283
Level 2 \| Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,609	3,103
Level 2 \| Municipal obligations
Securities owned
Securities owned, at fair value	28,393	25,190
Level 2 \| Convertible bonds
Securities owned
Securities owned, at fair value	17,152	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,647
Level 2 \| Mortgage Backed Securities, Other
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4
Level 3
Securities owned
Securities owned, at fair value	31,804	30,701
Investments	0
Derivative contracts
Total	31,804	30,701
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	0	195
Total	0	195
Level 3 \| Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts		195
Level 3 \| Auction rate securities
Securities owned
Securities owned, at fair value	31,804	30,701
Level 3 \| Municipal obligations
Securities owned
Securities owned, at fair value		0
TBAs
Derivative contracts
Derivative contracts, total	526	15
Derivative contracts:
Derivative contracts	520	3
TBAs \| Level 2
Derivative contracts
Derivative contracts, total	526	15
Derivative contracts:
Derivative contracts	520	3
Future [Member]
Derivative contracts:
Derivative contracts	11	22
Future [Member] \| Level 1
Derivative contracts:
Derivative contracts	$ 11	$ 22

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