Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
ASSETS
Cash equivalents	$ 75,462,000	$ 16,242,000
Deposits with clearing organizations	27,386,000	26,437,000
Securities owned:
Securities owned	986,992,000	710,668,000
Investments	7,441,000
Securities purchased under agreements to resell	3,617,000	24,006,000
Derivative contracts:
Derivative contracts		1,663,000
Total	1,096,526,000	779,174,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	234,778,000	85,050,000
Derivative contracts:
Derivative contracts	4,130,000	2,024,000
Total	238,908,000	87,074,000
Future [Member]
Derivative contracts:
Derivative contracts	853,000	166,000
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	8,000	1,000
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	2,901,000	814,000
Derivative contracts:
Derivative contracts	3,015,000	1,212,000
ARS Purchase Commitments [Member]
Derivative contracts:
Derivative contracts		849,000
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	254,000	645,000
Investments [Member]
Securities owned:
Investments	168,000	158,000
Equity Securities [Member]
Securities owned:
Securities owned	35,473,000	31,174,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	37,346,000	41,689,000
Money Markets [Member]
Securities owned:
Securities owned	578,000	189,000
Auction Rate Securities [Member]
Securities owned:
Securities owned	107,170,000	84,926,000
U.S. Treasury Securities [Member]
Securities owned:
Securities owned	583,656,000	418,888,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	142,877,000	28,662,000
US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	13,911,000	38,269,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	6,000	12,000
Sovereign Obligations [Member]
Securities owned:
Securities owned	20,561,000	1,894,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	16,653,000
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	34,051,000	17,074,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	19,306,000	2,536,000
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	4,562,000	5,024,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	2,134,000	31,000
Municipal Notes [Member]
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value		516,000
Municipal Bonds [Member]
Securities owned:
Securities owned	141,996,000	56,750,000
Convertible Bonds [Member]
Securities owned:
Securities owned	45,034,000	56,480,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	16,456,000	11,604,000
Level 1 [Member]
ASSETS
Cash equivalents	75,462,000	16,242,000
Deposits with clearing organizations	27,386,000	26,437,000
Securities owned:
Securities owned	621,449,000	456,129,000
Derivative contracts:
Total	724,297,000	498,808,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	180,223,000	70,351,000
Derivative contracts:
Derivative contracts	861,000	167,000
Total	181,084,000	70,518,000
Level 1 [Member] | Future [Member]
Derivative contracts:
Derivative contracts	853,000	166,000
Level 1 [Member] | Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	8,000	1,000
Level 1 [Member] | Equity Securities [Member]
Securities owned:
Securities owned	35,473,000	31,174,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	37,346,000	41,689,000
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities owned	578,000	189,000
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities owned	583,656,000	418,888,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	142,877,000	28,662,000
Level 1 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	1,742,000	5,878,000
Level 2 [Member]
Securities owned:
Securities owned	258,337,000	169,569,000
Securities purchased under agreements to resell	3,617,000	24,006,000
Derivative contracts:
Derivative contracts		814,000
Total	264,855,000	194,389,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	54,555,000	14,699,000
Derivative contracts:
Derivative contracts	3,015,000	1,212,000
Total	57,570,000	15,911,000
Level 2 [Member] | To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	2,901,000	814,000
Derivative contracts:
Derivative contracts	3,015,000	1,212,000
Level 2 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	12,169,000	32,391,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	6,000	12,000
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities owned	20,561,000	1,894,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	16,653,000
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	34,051,000	17,074,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	19,306,000	2,536,000
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	4,562,000	5,024,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	2,134,000	31,000
Level 2 [Member] | Municipal Notes [Member]
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value		516,000
Level 2 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	141,960,000	56,706,000
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities owned	45,034,000	56,480,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	16,456,000	11,604,000
Level 3 [Member]
Securities owned:
Securities owned	107,206,000	84,970,000
Derivative contracts:
Derivative contracts		849,000
Total	107,374,000	85,977,000
Derivative contracts:
Derivative contracts	254,000	645,000
Total	254,000	645,000
Level 3 [Member] | ARS Purchase Commitments [Member]
Derivative contracts:
Derivative contracts		849,000
Level 3 [Member] | Investments [Member]
Securities owned:
Investments	168,000	158,000
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities owned	107,170,000	84,926,000
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	254,000	645,000
Level 3 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	36,000	44,000
OMHHF
Derivative contracts:
Securities owned	$ 560,000	$ 3,600,000