Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
ASSETS
Deposits with clearing organizations	$ 25,899	$ 23,991
Securities owned
Securities owned, at fair value	605,330	610,517
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	605,330	610,517
Investments	4,440	4,181
Investments	4,821	4,836
Derivative contracts
Total	635,752	638,704
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	360,486	126,171
Derivative contracts:
Derivative contracts	417	220
Total	360,903	126,391
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	65	195
Equity Securities [Member]
Securities owned
Securities owned, at fair value	41,364	36,554
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	22,187	17,892
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	413	200
Auction rate securities
Securities owned
Securities owned, at fair value	31,470	30,701
U.S. Treasury securities
Securities owned
Securities owned, at fair value	427,489	448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	324,612	93,261
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	18,339	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5	9
Sovereign obligations
Securities owned
Securities owned, at fair value	409	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,609	623
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	21,227	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,199	5,283
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	5,541	3,103
Municipal obligations
Securities owned
Securities owned, at fair value	40,296	25,190
Convertible bonds
Securities owned
Securities owned, at fair value	18,782	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,873
Mortgage Backed Securities, Other
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1
Level 1
ASSETS
Deposits with clearing organizations	25,899	23,991
Securities owned
Securities owned, at fair value	469,266	485,066
Derivative contracts
Total	495,165	509,057
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	346,799	111,153
Derivative contracts:
Derivative contracts	296	22
Total	347,095	111,175
Level 1 \| Corporate equities
Securities owned
Securities owned, at fair value	41,364	36,554
Level 1 \| Money markets
Securities owned
Securities owned, at fair value	413	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	427,489	448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	324,612	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	22,187	17,892
Level 2
Securities owned
Securities owned, at fair value	104,594	94,750
Investments	4,440	4,181
Derivative contracts
Total	109,117	98,946
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,687	15,018
Derivative contracts:
Derivative contracts	56	3
Total	13,743	15,021
Level 2 \| Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	0	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	18,339	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5	9
Level 2 \| Sovereign obligations
Securities owned
Securities owned, at fair value	409	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,609	623
Level 2 \| Corporate debt and other obligations
Securities owned
Securities owned, at fair value	21,227	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,199	5,283
Level 2 \| Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	5,541	3,103
Level 2 \| Municipal obligations
Securities owned
Securities owned, at fair value	40,296	25,190
Level 2 \| Convertible bonds
Securities owned
Securities owned, at fair value	18,782	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,873
Level 2 \| Mortgage Backed Securities, Other
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1
Level 3
Securities owned
Securities owned, at fair value	31,470	30,701
Investments	0
Derivative contracts
Total	31,470	30,701
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	65	195
Total	65	195
Level 3 \| Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	65	195
Level 3 \| Auction rate securities
Securities owned
Securities owned, at fair value	31,470	30,701
Level 3 \| Municipal obligations
Securities owned
Securities owned, at fair value		0
TBAs
Derivative contracts
Derivative contracts, total	83	15
Derivative contracts:
Derivative contracts	56	3
TBAs \| Level 2
Derivative contracts
Derivative contracts, total	83	15
Derivative contracts:
Derivative contracts	56	3
Future [Member]
Derivative contracts:
Derivative contracts	296	22
Future [Member] \| Level 1
Derivative contracts:
Derivative contracts	$ 296	$ 22

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