Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
ASSETS
Deposits with clearing organizations	$ 36,593	$ 25,118
Securities owned
Securities owned, at fair value	309,346	799,719
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	309,346	799,719
Investments	4,979
Derivative contracts
Total	346,577	824,837
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	34,639	100,571
Derivative contracts:
Derivative contracts	2,722	1,414
Total	37,361	101,985
Equity Securities [Member]
Securities owned
Securities owned, at fair value	25,582	32,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,552	21,978
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	917	781
Auction rate securities
Securities owned
Securities owned, at fair value	26,336	25,314
U.S. Treasury securities
Securities owned
Securities owned, at fair value	156,304	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,679	52,882
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	12,496	35,891
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	8	18
Sovereign obligations
Securities owned
Securities owned, at fair value	1,776	11,405
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,389	6,405
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	16,463	8,310
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,131	664
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,243	2,697
Municipal obligations
Securities owned
Securities owned, at fair value	45,448	40,260
Convertible bonds
Securities owned
Securities owned, at fair value	21,781	29,816
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3,880	0
Level 1
ASSETS
Deposits with clearing organizations	36,593	25,118
Securities owned
Securities owned, at fair value	185,445	665,943
Derivative contracts
Total	222,038	691,061
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	24,231	74,860
Derivative contracts:
Derivative contracts	1,192	267
Total	25,423	75,127
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	25,582	32,215
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,552	21,978
Level 1 | Money markets
Securities owned
Securities owned, at fair value	917	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	156,304	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,679	52,882
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	2,642	19,917
Level 2
Securities owned
Securities owned, at fair value	123,901	133,776
Derivative contracts
Total	124,539	133,776
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,408	25,711
Derivative contracts:
Derivative contracts	1,530	1,147
Total	11,938	26,858
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	26,336	25,314
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	9,854	15,974
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	8	18
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	1,776	11,405
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,389	6,405
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	16,463	8,310
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,131	664
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,243	2,697
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	45,448	40,260
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	21,781	29,816
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3,880	0
Level 3
Securities owned
Securities owned, at fair value	0	0
Derivative contracts
Total	0	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	0	0
Total	0	0
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	0	0
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		0
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	887	1,023
Auction Rate Securities Purchase Commitment | Level 2
Derivative contracts:
Derivative contracts	887	1,023
Auction Rate Securities Purchase Commitment | Level 3
Derivative contracts:
Derivative contracts	0	0
TBAs
Derivative contracts
Derivative contracts, total	638
Derivative contracts:
Derivative contracts	643	124
TBAs | Level 2
Derivative contracts
Derivative contracts, total	638
Derivative contracts:
Derivative contracts	643	124
Future [Member]
Derivative contracts:
Derivative contracts	1,161	267
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	1,161	267
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	31	0
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts	$ 31	0
Foreign exchange forward contracts | Level 2
Derivative contracts:
Derivative contracts		$ 0