Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Sep. 30, 2016	Dec. 31, 2015
ASSETS
Cash equivalents	$ 47,897,000	$ 13,000,000
Deposits with clearing organizations	23,989,000	31,456,000
Securities owned:
Securities owned	1,028,496,000	735,393,000
Investments	7,036,000
Loans held for sale		60,234,000
Securities purchased under agreements to resell		206,499,000
Derivative contracts:
Derivative contracts	1,827,000	15,609,000
Total	1,102,398,000	1,062,348,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	295,507,000	126,493,000
Derivative contracts:
Derivative contracts	1,413,000	14,162,000
Total	296,920,000	140,655,000
Future [Member]
Derivative contracts:
Derivative contracts	209,000	249,000
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	1,000	2,000
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	869,000	6,448,000
Derivative contracts:
Derivative contracts	927,000	11,619,000
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts	53,000	9,161,000
Derivative contracts:
Derivative contracts		923,000
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	905,000
Derivative contracts:
Derivative contracts	276,000	1,369,000
Investments [Member]
Securities owned:
Investments	189,000	157,000
Equity Securities [Member]
Securities owned:
Securities owned	37,057,000	34,475,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	53,262,000	41,378,000
Money Markets [Member]
Securities owned:
Securities owned	1,142,000	35,000
Auction Rate Securities [Member]
Securities owned:
Securities owned	90,009,000	86,802,000
U.S. Treasury Securities [Member]
Securities owned:
Securities owned	673,925,000	436,533,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	220,346,000	75,653,000
US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	46,933,000	71,416,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	33,000	15,000
Sovereign Obligations [Member]
Securities owned:
Securities owned	3,351,000	1,665,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	2,658,000	1,817,000
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	32,262,000	16,138,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	8,708,000	1,652,000
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	4,551,000	3,504,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	46,000	27,000
Municipal Bonds [Member]
Securities owned:
Securities owned	85,892,000	30,132,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	0	0
Convertible Bonds [Member]
Securities owned:
Securities owned	53,374,000	54,693,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	10,454,000	5,951,000
Level 1 [Member]
ASSETS
Cash equivalents	47,897,000	13,000,000
Deposits with clearing organizations	23,989,000	31,456,000
Securities owned:
Securities owned	719,832,000	496,283,000
Derivative contracts:
Total	791,718,000	540,739,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	273,608,000	117,031,000
Derivative contracts:
Derivative contracts	210,000	251,000
Total	273,818,000	117,282,000
Level 1 [Member] | Future [Member]
Derivative contracts:
Derivative contracts	209,000	249,000
Level 1 [Member] | Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	1,000	2,000
Level 1 [Member] | Equity Securities [Member]
Securities owned:
Securities owned	37,057,000	34,475,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	53,262,000	41,378,000
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities owned	1,142,000	35,000
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities owned	673,925,000	436,533,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	220,346,000	75,653,000
Level 1 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	7,708,000	25,240,000
Level 2 [Member]
Securities owned:
Securities owned	218,630,000	152,227,000
Loans held for sale		60,234,000
Securities purchased under agreements to resell		206,499,000
Derivative contracts:
Derivative contracts	869,000	6,448,000
Total	219,499,000	425,408,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	21,899,000	9,462,000
Derivative contracts:
Derivative contracts	927,000	11,619,000
Total	22,826,000	21,081,000
Level 2 [Member] | To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	869,000	6,448,000
Derivative contracts:
Derivative contracts	927,000	11,619,000
Level 2 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	39,225,000	46,176,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	33,000	15,000
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities owned	3,351,000	1,665,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	2,658,000	1,817,000
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	32,262,000	16,138,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	8,708,000	1,652,000
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	4,551,000	3,504,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	46,000	27,000
Level 2 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	85,867,000	30,051,000
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities owned	53,374,000	54,693,000
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	10,454,000	5,951,000
Level 3 [Member]
Securities owned:
Securities owned	90,034,000	86,883,000
Derivative contracts:
Derivative contracts	958,000	9,161,000
Total	91,181,000	96,201,000
Derivative contracts:
Derivative contracts	276,000	2,292,000
Total	276,000	2,292,000
Level 3 [Member] | Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts	53,000	9,161,000
Derivative contracts:
Derivative contracts		923,000
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	905,000
Level 3 [Member] | Investments [Member]
Securities owned:
Investments	189,000	157,000
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities owned	90,009,000	86,802,000
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	276,000	1,369,000
Level 3 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	25,000	81,000
OMHHF
Derivative contracts:
Securities owned	$ 563,000	$ 562,000