Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2017	Dec. 31, 2017
ASSETS
Cash equivalents	$ 10,500,000			$ 10,490,000
Deposits with clearing organizations	31,135,000			34,293,000
Securities owned
Securities owned, at fair value	1,020,404,000			926,597,000
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	1,020,404,000			926,597,000
Investments	7,632,000
Derivative contracts
Total	1,065,018,000			972,265,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	162,042,000			94,486,000
Derivative contracts:
Derivative contracts	3,884,000			1,388,000
Total	165,926,000			95,874,000
Future [Member]
Derivative contracts:
Derivative contracts	942,000			766,000
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	5,000
TBAs
Derivative contracts
Derivative contracts, total	2,815,000			716,000
Derivative contracts:
Derivative contracts	2,715,000			614,000
Investments
Securities owned
Investments	164,000			169,000
Equity Securities [Member]
Securities owned
Securities owned, at fair value	33,112,000			34,067,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	26,978,000			25,393,000
Money Market Funds [Member]
Securities owned
Securities owned, at fair value				383,000
Auction rate securities
Securities owned
Securities owned, at fair value	90,223,000			111,853,000
U.S. Treasury securities
Securities owned
Securities owned, at fair value	690,988,000			640,337,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	115,465,000			53,425,000
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	16,447,000			9,905,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,000			13,000
Sovereign obligations
Securities owned
Securities owned, at fair value	204,000			608,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value				1,179,000
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	24,813,000			12,538,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,289,000			4,357,000
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	6,870,000			4,037,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,808,000			10,000
Municipal obligations
Securities owned
Securities owned, at fair value	116,612,000			89,653,000
Convertible bonds
Securities owned
Securities owned, at fair value	41,135,000			23,216,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,496,000			10,109,000
Level 1
ASSETS
Cash equivalents	10,500,000			10,490,000
Deposits with clearing organizations	31,135,000			34,293,000
Securities owned
Securities owned, at fair value	732,641,000			677,798,000
Derivative contracts
Total	774,276,000			722,581,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	142,443,000			78,818,000
Derivative contracts:
Derivative contracts	947,000			766,000
Total	143,390,000			79,584,000
Level 1 | Future [Member]
Derivative contracts:
Derivative contracts	942,000			766,000
Level 1 | Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	5,000
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	33,112,000			34,067,000
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	26,978,000			25,393,000
Level 1 | Money markets
Securities owned
Securities owned, at fair value				383,000
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	690,988,000			640,337,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	115,465,000			53,425,000
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	8,541,000			3,011,000
Level 2
Securities owned
Securities owned, at fair value	215,142,000			161,366,000
Derivative contracts
Total	217,957,000			162,082,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	19,599,000			15,668,000
Derivative contracts:
Derivative contracts	2,806,000			614,000
Total	22,405,000			16,282,000
Level 2 | TBAs
Derivative contracts
Derivative contracts, total	2,815,000			716,000
Derivative contracts:
Derivative contracts	2,715,000			614,000
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	17,602,000			24,455,000
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	7,906,000			6,894,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,000			13,000
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	204,000			608,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value				1,179,000
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	24,813,000			12,538,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,289,000			4,357,000
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	6,870,000			4,037,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	6,808,000			10,000
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	116,612,000			89,618,000
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	41,135,000			23,216,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,496,000			10,109,000
Level 3
Securities owned
Securities owned, at fair value	72,621,000			87,433,000
Derivative contracts
Total	72,785,000			87,602,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	131,000			8,000
Total	131,000			8,000
Level 3 | Investments
Securities owned
Investments	164,000			169,000
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	72,621,000			87,398,000
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value				35,000
Auction rate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers, Net	17,600,000	$ 0	$ 0
ARS purchase commitments
Derivative contracts:
Derivative contracts	222,000			8,000
ARS purchase commitments | Level 2
Derivative contracts:
Derivative contracts	91,000
ARS purchase commitments | Level 3
Derivative contracts:
Derivative contracts	$ 131,000			$ 8,000