Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
ASSETS
Cash equivalents	$ 10,490	$ 16,242
Deposits with clearing organizations	29,448	26,437
Securities owned:
Securities owned	1,037,463	710,668
Investments	7,473
Securities purchased under agreements to resell		24,006
Derivative contracts:
Derivative contracts		1,663
Total	1,078,907	779,174
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	366,581	85,050
Derivative contracts:
Derivative contracts	2,169	2,024
Total	368,750	87,074
OMHHF
Derivative contracts:
Securities owned		3,600
Future [Member]
Derivative contracts:
Derivative contracts	663	166
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts		1
TBAs and Other TBAs [Member]
Derivative contracts:
Derivative contracts	1,339	814
Derivative contracts:
Derivative contracts	1,230	1,212
ARS Purchase Commitments [Member]
Derivative contracts:
Derivative contracts		849
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	276	645
Investments [Member]
Securities owned:
Investments	167	158
Equity Securities [Member]
Securities owned:
Securities owned	46,239	31,174
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	42,150	41,689
Money Markets [Member]
Securities owned:
Securities owned	174	189
Auction Rate Securities [Member]
Securities owned:
Securities owned	106,784	84,926
U.S. Treasury Securities [Member]
Securities owned:
Securities owned	731,902	418,888
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	272,071	28,662
US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	20,521	38,269
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	3	12
Sovereign Obligations [Member]
Securities owned:
Securities owned	18,439	1,894
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	18,453
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	18,181	17,074
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	10,459	2,536
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	2,483	5,024
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	13	31
Municipal Notes [Member]
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value		516
Municipal Bonds [Member]
Securities owned:
Securities owned	42,921	56,750
Convertible Bonds [Member]
Securities owned:
Securities owned	49,819	56,480
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	23,432	11,604
Level 1 [Member]
ASSETS
Cash equivalents	10,490	16,242
Deposits with clearing organizations	29,448	26,437
Securities owned:
Securities owned	788,345	456,129
Derivative contracts:
Total	828,283	498,808
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	314,221	70,351
Derivative contracts:
Derivative contracts	663	167
Total	314,884	70,518
Level 1 [Member] | Future [Member]
Derivative contracts:
Derivative contracts	663	166
Level 1 [Member] | Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts		1
Level 1 [Member] | Equity Securities [Member]
Securities owned:
Securities owned	46,239	31,174
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	42,150	41,689
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities owned	174	189
Level 1 [Member] | U.S. Treasury Securities [Member]
Securities owned:
Securities owned	731,902	418,888
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	272,071	28,662
Level 1 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	10,030	5,878
Level 2 [Member]
Securities owned:
Securities owned	142,299	169,569
Securities purchased under agreements to resell		24,006
Derivative contracts:
Derivative contracts		814
Total	143,638	194,389
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	52,360	14,699
Derivative contracts:
Derivative contracts	1,230	1,212
Total	53,590	15,911
Level 2 [Member] | TBAs and Other TBAs [Member]
Derivative contracts:
Derivative contracts	1,339	814
Derivative contracts:
Derivative contracts	1,230	1,212
Level 2 [Member] | US Government Agencies Debt Securities [Member]
Securities owned:
Securities owned	10,491	32,391
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	3	12
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities owned	18,439	1,894
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	18,453
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities owned	18,181	17,074
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	10,459	2,536
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities owned	2,483	5,024
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	13	31
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
Level 2 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	42,886	56,706
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities owned	49,819	56,480
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold but not yet purchased, at fair value	23,432	11,604
Level 3 [Member]
Securities owned:
Securities owned	106,819	84,970
Derivative contracts:
Derivative contracts		849
Total	106,986	85,977
Derivative contracts:
Derivative contracts	276	645
Total	276	645
Level 3 [Member] | ARS Purchase Commitments [Member]
Derivative contracts:
Derivative contracts		849
Level 3 [Member] | Investments [Member]
Securities owned:
Investments	167	158
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities owned	106,784	84,926
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	276	645
Level 3 [Member] | Municipal Bonds [Member]
Securities owned:
Securities owned	$ 35	$ 44