Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
ASSETS
Cash equivalents	$ 13,449	$ 13,000
Deposits with clearing organizations	29,981	31,456
Securities owned:
Securities owned	1,036,782	735,393
Investments	7,262
Loans held for sale	13,532	60,234
Securities purchased under agreements to resell		206,499
Derivative contracts:
Derivative contracts	16,375	15,609
Total	1,110,280	1,062,348
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	270,952	126,493
Derivative contracts:
Derivative contracts	13,101	14,162
Total	284,053	140,655
Future [Member]
Derivative contracts:
Derivative contracts	709	249
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	17	2
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	811	6,448
Derivative contracts:
Derivative contracts	11,816	11,619
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts	14,024	9,161
Derivative contracts:
Derivative contracts		923
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,540
Derivative contracts:
Derivative contracts	559	1,369
Investments [Member]
Securities owned:
Investments	161	157
Equity Securities [Member]
Securities owned:
Securities owned	44,964	34,475
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	39,768	41,378
Money Markets [Member]
Securities owned:
Securities owned	132	35
Auction Rate Securities [Member]
Securities owned:
Securities owned	84,185	86,802
U.S. Treasury Securities [Member]
Securities owned:
Securities owned	715,404	436,533
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	202,838	75,653
US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	30,545	71,416
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	13	15
Sovereign Obligations [Member]
Securities owned:
Securities owned	3,374	1,665
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	1,152	1,817
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	22,823	16,138
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	11,166	1,652
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	4,151	3,504
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	27	27
Municipal Bonds [Member]
Securities owned:
Securities owned	80,220	30,132
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	50,984	54,693
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	15,988	5,951
Level 1 [Member]
ASSETS
Cash equivalents	13,449	13,000
Deposits with clearing organizations	29,981	31,456
Securities owned:
Securities owned	761,500	496,283
Derivative contracts:
Total	804,930	540,739
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	242,606	117,031
Derivative contracts:
Derivative contracts	726	251
Total	243,332	117,282
Level 1 [Member] | Future [Member]
Derivative contracts:
Derivative contracts	709	249
Level 1 [Member] | Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	17	2
Level 1 [Member] | Equity Securities [Member]
Securities owned:
Securities owned	44,964	34,475
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	39,768	41,378
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities owned	132	35
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities owned	715,404	436,533
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	202,838	75,653
Level 1 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	1,000	25,240
Level 2 [Member]
Securities owned:
Securities owned	191,012	152,227
Loans held for sale	13,532	60,234
Securities purchased under agreements to resell		206,499
Derivative contracts:
Derivative contracts	811	6,448
Total	205,355	425,408
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	28,346	9,462
Derivative contracts:
Derivative contracts	11,816	11,619
Total	40,162	21,081
Level 2 [Member] | To Be Announced Security [Member]
Derivative contracts:
Derivative contracts	811	6,448
Derivative contracts:
Derivative contracts	11,816	11,619
Level 2 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	29,545	46,176
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	13	15
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities owned	3,374	1,665
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	1,152	1,817
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	22,823	16,138
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	11,166	1,652
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	4,151	3,504
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	27	27
Level 2 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	80,135	30,051
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities owned	50,984	54,693
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	15,988	5,951
Level 3 [Member]
Securities owned:
Securities owned	84,270	86,883
Derivative contracts:
Derivative contracts	15,564	9,161
Total	99,995	96,201
Derivative contracts:
Derivative contracts	559	2,292
Total	559	2,292
Level 3 [Member] | Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts	14,024	9,161
Derivative contracts:
Derivative contracts		923
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,540
Level 3 [Member] | Investments [Member]
Securities owned:
Investments	161	157
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities owned	84,185	86,802
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	559	1,369
Level 3 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	$ 85	$ 81