Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
ASSETS
Cash equivalents		$ 10,500
Deposits with clearing organizations	$ 25,118	34,599
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		837,584
Securities owned
Securities owned, at fair value	799,719	837,584
Investments	5,816
Derivative contracts
Total	824,837	887,657
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	100,571	85,446
Investments	0	0
Derivative contracts:
Derivative contracts	1,414	6,780
Total	101,985	92,226
Investments
Securities owned
Investments		101
Equity Securities [Member]
Securities owned
Securities owned, at fair value	32,215	28,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	21,978
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	781	7
Auction rate securities
Securities owned
Securities owned, at fair value	25,314	37,952
Corporate Equities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		14,774
Money Markets [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		0
U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		657,208
Securities owned
Securities owned, at fair value	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	52,882	53,646
US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	35,891	7,306
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		3
US Government Agencies Debt Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18
Sovereign obligations
Securities owned
Securities owned, at fair value	11,405	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,405	78
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	8,310	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	664	7,236
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,697	2,486
Municipal obligations
Securities owned
Securities owned, at fair value	40,260	52,261
Convertible bonds
Securities owned
Securities owned, at fair value	29,816	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,624	9,709
Level 1
ASSETS
Cash equivalents		10,500
Deposits with clearing organizations	25,118	34,599
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		686,242
Securities owned
Securities owned, at fair value	665,943
Derivative contracts
Total	691,061	731,341
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	74,860	68,420
Investments	0	0
Derivative contracts:
Derivative contracts	267	811
Total	75,127	69,231
Level 1 | Equity Securities [Member]
Securities owned
Securities owned, at fair value	32,215	28,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	21,978
Level 1 | Money Market Funds [Member]
Securities owned
Securities owned, at fair value	781	7
Level 1 | Auction rate securities
Securities owned
Securities owned, at fair value	0
Level 1 | Corporate Equities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		14,774
Level 1 | Money Markets [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		0
Level 1 | U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		657,208
Securities owned
Securities owned, at fair value	613,030
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	52,882	53,646
Level 1 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	19,917	812
Level 2
ASSETS
Deposits with clearing organizations	0
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		129,643
Securities owned
Securities owned, at fair value	133,776
Derivative contracts
Total	133,776	134,516
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	25,711	17,026
Derivative contracts:
Derivative contracts	1,147	5,969
Total	26,858	22,995
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	25,314	16,253
Level 2 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	15,974	6,494
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18	3
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	11,405	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,405	78
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	8,310	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	664	7,236
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,697	2,486
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	40,260	52,261
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	29,816	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,624	9,709
Level 3
ASSETS
Deposits with clearing organizations	0
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		21,699
Securities owned
Securities owned, at fair value	0
Derivative contracts
Total	0	21,800
Derivative contracts:
Derivative contracts	0	0
Total	0	0
Level 3 | Investments
Securities owned
Investments		101
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	0	21,699
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	0	0
TBAs
Derivative contracts
Derivative contracts, total		4,873
Derivative contracts:
Derivative contracts	124	4,873
TBAs | Level 2
Derivative contracts
Derivative contracts, total		4,873
Derivative contracts:
Derivative contracts	124	4,873
ARS purchase commitments
Derivative contracts:
Derivative contracts	1,023	1,096
ARS purchase commitments | Level 2
Derivative contracts:
Derivative contracts	1,023
ARS purchase commitments | Level 3
Derivative contracts:
Derivative contracts	0	0
Future [Member]
Derivative contracts:
Derivative contracts	267	807
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	$ 267	807
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts		4
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts		$ 4