Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
ASSETS
Deposits with clearing organizations	$ 35,344	$ 25,118
Securities owned
Securities owned, at fair value	646,680	799,719
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	646,680	799,719
Investments	3,961
Investments	5,489	5,816
Derivative contracts
Total	686,260	824,837
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	222,449	100,571
Derivative contracts:
Derivative contracts	528	1,414
Total	222,977	101,985
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	192	1,023
Equity Securities [Member]
Securities owned
Securities owned, at fair value	32,121	32,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	22,639	21,978
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	1,470	781
Auction rate securities
Securities owned
Securities owned, at fair value	30,701	25,314
U.S. Treasury securities
Securities owned
Securities owned, at fair value	470,683	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	181,419	52,882
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	14,787	35,891
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,972	18
Sovereign obligations
Securities owned
Securities owned, at fair value	207	11,405
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	936	6,405
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	27,111	8,310
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,049	664
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,963	2,697
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		18,624
Municipal obligations
Securities owned
Securities owned, at fair value	46,645	40,260
Convertible bonds
Securities owned
Securities owned, at fair value	19,992	29,816
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,434
Level 1
ASSETS
Deposits with clearing organizations	35,344	25,118
Securities owned
Securities owned, at fair value	504,274	665,943
Derivative contracts
Total	539,618	691,061
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	204,058	74,860
Derivative contracts:
Derivative contracts	116	267
Total	204,174	75,127
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	32,121	32,215
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	22,639	21,978
Level 1 | Money markets
Securities owned
Securities owned, at fair value	1,470	781
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	470,683	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	181,419	52,882
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	0	19,917
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 2
Securities owned
Securities owned, at fair value	111,705	133,776
Derivative contracts
Total	115,941	133,776
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,391	25,711
Derivative contracts:
Derivative contracts	220	1,147
Total	18,611	26,858
Level 2 | Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	0	1,023
Level 2 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	0	25,314
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	14,787	15,974
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,972	18
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	207	11,405
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	936	6,405
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	27,111	8,310
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,049	664
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,963	2,697
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		18,624
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	46,645	40,260
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	19,992	29,816
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,434
Level 3
Securities owned
Securities owned, at fair value	30,701	0
Investments	0
Derivative contracts
Total	30,701	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	192	0
Total	192	0
Level 3 | Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	192	0
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	30,701	0
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		0
TBAs
Derivative contracts
Derivative contracts, total	275
Derivative contracts:
Derivative contracts	220	124
TBAs | Level 2
Derivative contracts
Derivative contracts, total	275
Derivative contracts:
Derivative contracts	220	124
Future [Member]
Derivative contracts:
Derivative contracts	116	267
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	$ 116	$ 267