Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
ASSETS
Deposits with clearing organizations	$ 29,994	$ 23,991
Securities owned
Securities owned, at fair value	544,062	610,517
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	544,062	610,517
Investments	4,709	4,181
Investments	4,960	4,836
Derivative contracts
Derivative contracts, total	180
Total	578,945	638,704
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	112,838	126,171
Derivative contracts:
Derivative contracts	747	220
Total	113,585	126,391
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	66	195
Equity Securities [Member]
Securities owned
Securities owned, at fair value	48,540	36,554
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	31,485	17,892
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	408	200
Auction rate securities
Securities owned
Securities owned, at fair value	31,422	30,701
U.S. Treasury securities
Securities owned
Securities owned, at fair value	369,604	448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	60,021	93,261
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	15,060	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7	9
Sovereign obligations
Securities owned
Securities owned, at fair value	8,713	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,187	623
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	20,621	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,398	5,283
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	3,112	3,103
Municipal obligations
Securities owned
Securities owned, at fair value	23,270	25,190
Convertible bonds
Securities owned
Securities owned, at fair value	23,312	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,739
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	29,994	23,991
Securities owned
Securities owned, at fair value	418,552	485,066
Derivative contracts
Derivative contracts, total	144
Total	448,690	509,057
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	91,506	111,153
Derivative contracts:
Derivative contracts	646	22
Total	92,152	111,175
Level 1 \| Corporate equities
Securities owned
Securities owned, at fair value	48,540	36,554
Level 1 \| Money markets
Securities owned
Securities owned, at fair value	408	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	369,604	448,312
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	60,021	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	31,485	17,892
Level 2
Securities owned
Securities owned, at fair value	94,088	94,750
Investments	4,709	4,181
Derivative contracts
Derivative contracts, total	36
Total	98,833	98,946
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	21,332	15,018
Derivative contracts:
Derivative contracts	35	3
Total	21,367	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	15,060	24,616
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7	9
Level 2 \| Sovereign obligations
Securities owned
Securities owned, at fair value	8,713	367
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,187	623
Level 2 \| Corporate debt and other obligations
Securities owned
Securities owned, at fair value	20,621	23,977
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,398	5,283
Level 2 \| Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	3,112	3,103
Level 2 \| Municipal obligations
Securities owned
Securities owned, at fair value	23,270	25,190
Level 2 \| Convertible bonds
Securities owned
Securities owned, at fair value	23,312	17,497
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,739
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	31,422	30,701
Investments	0
Derivative contracts
Total	31,422	30,701
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	66	195
Total	66	195
Level 3 \| Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	66	195
Level 3 \| Auction rate securities
Securities owned
Securities owned, at fair value	31,422	30,701
Level 3 \| Municipal obligations
Securities owned
Securities owned, at fair value		0
TBAs
Derivative contracts
Derivative contracts, total	36	15
Derivative contracts:
Derivative contracts	35	3
TBAs \| Level 2
Derivative contracts
Derivative contracts, total	36	15
Derivative contracts:
Derivative contracts	35	3
Future [Member]
Derivative contracts:
Derivative contracts	11	22
Future [Member] \| Level 1
Derivative contracts:
Derivative contracts	11	$ 22
Purchase Commitment
Derivative contracts
Derivative contracts, total	144
Derivative contracts:
Derivative contracts	635
Purchase Commitment \| Level 1
Derivative contracts
Derivative contracts, total	144
Derivative contracts:
Derivative contracts	$ 635

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