Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
ASSETS
Deposits with clearing organizations	$ 35,125	$ 25,118
Securities owned
Securities owned, at fair value	502,029	799,719
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	502,029	799,719
Investments	4,716
Derivative contracts
Total	537,186	824,837
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	356,589	100,571
Derivative contracts:
Derivative contracts	786	1,414
Total	357,375	101,985
Equity Securities [Member]
Securities owned
Securities owned, at fair value	30,734	32,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	26,415	21,978
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	1,526	781
Auction rate securities
Securities owned
Securities owned, at fair value	29,566	25,314
U.S. Treasury securities
Securities owned
Securities owned, at fair value	317,509	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	304,632	52,882
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	20,503	35,891
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,361	18
Sovereign obligations
Securities owned
Securities owned, at fair value	4,675	11,405
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3,561	6,405
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	16,825	8,310
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,605	664
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,437	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	60,403	40,260
Convertible bonds
Securities owned
Securities owned, at fair value	17,851	29,816
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,015
Level 1
ASSETS
Deposits with clearing organizations	35,125	25,118
Securities owned
Securities owned, at fair value	366,113	665,943
Derivative contracts
Total	401,238	691,061
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	332,815	74,860
Derivative contracts:
Derivative contracts	414	267
Total	333,229	75,127
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	30,734	32,215
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	26,415	21,978
Level 1 | Money markets
Securities owned
Securities owned, at fair value	1,526	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	317,509	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	304,632	52,882
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	14,106	19,917
Level 1 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	2,238
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,768
Level 2
Securities owned
Securities owned, at fair value	106,350	133,776
Derivative contracts
Total	106,382	133,776
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	23,774	25,711
Derivative contracts:
Derivative contracts	40	1,147
Total	23,814	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	0	25,314
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	6,397	15,974
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,361	18
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	4,675	11,405
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3,561	6,405
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	14,587	8,310
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,837	664
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,437	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	60,403	40,260
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	17,851	29,816
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,015
Level 3
Securities owned
Securities owned, at fair value	29,566	0
Derivative contracts
Total	29,566	0
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	332	0
Total	332	0
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	29,566	0
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		0
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	332	1,023
Auction Rate Securities Purchase Commitment | Level 2
Derivative contracts:
Derivative contracts	0	1,023
Auction Rate Securities Purchase Commitment | Level 3
Derivative contracts:
Derivative contracts	332	0
TBAs
Derivative contracts
Derivative contracts, total	32
Derivative contracts:
Derivative contracts	40	124
TBAs | Level 2
Derivative contracts
Derivative contracts, total	32
Derivative contracts:
Derivative contracts	40	124
Future [Member]
Derivative contracts:
Derivative contracts	397	267
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	397	$ 267
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	17
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts	$ 17