Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
ASSETS
Cash equivalents	$ 10,490	$ 16,242
Deposits with clearing organizations	34,293	26,437
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		710,668
Securities owned
Securities owned, at fair value	926,597	707,108
Investments	7,617
Securities purchased under agreements to resell		24,006
Derivative contracts
Derivative contracts, total		1,663
Total	972,265	779,174
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	94,486	85,050
Derivative contracts:
Derivative contracts	1,388	2,024
Total	95,874	87,074
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts		1
TBAs
Derivative contracts
Derivative contracts, total	716	814
Derivative contracts:
Derivative contracts	614	1,212
ARS purchase commitments
Derivative contracts
Derivative contracts, total		849
Derivative contracts:
Derivative contracts	8	645
Future [Member]
Derivative contracts:
Derivative contracts	766	166
Investments
Securities owned
Investments	169	158	[1]
Equity Securities [Member]
Securities owned
Securities owned, at fair value	34,067	31,174
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	25,393
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	383	189
Auction rate securities
Securities owned
Securities owned, at fair value	111,853	84,926
Corporate Equities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		41,689
U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		418,888
Securities owned
Securities owned, at fair value	640,337
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	53,425	28,662
US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	9,905	38,269
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		12
US Government Agencies Debt Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13
Sovereign obligations
Securities owned
Securities owned, at fair value	608	1,894
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,179
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	12,538	17,074
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,357	2,536
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	4,037	5,024
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10	31
Municipal obligations
Securities owned
Securities owned, at fair value	89,653	56,750
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		516
Convertible bonds
Securities owned
Securities owned, at fair value	23,216	56,480
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,109	11,604
Level 1
ASSETS
Cash equivalents	10,490	16,242
Deposits with clearing organizations	34,293	26,437
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		456,129
Securities owned
Securities owned, at fair value	677,798
Derivative contracts
Derivative contracts, total		0
Total	722,581	498,808
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	78,818	70,351
Derivative contracts:
Derivative contracts	766	167
Total	79,584	70,518
Level 1 | Foreign exchange forward contracts
Derivative contracts:
Derivative contracts		1
Level 1 | Future [Member]
Derivative contracts:
Derivative contracts	766	166
Level 1 | Equity Securities [Member]
Securities owned
Securities owned, at fair value	34,067	31,174
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	25,393
Level 1 | Money Market Funds [Member]
Securities owned
Securities owned, at fair value	383	189
Level 1 | Auction rate securities
Securities owned
Securities owned, at fair value	0
Level 1 | Corporate Equities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		41,689
Level 1 | U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		418,888
Securities owned
Securities owned, at fair value	640,337
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	53,425	28,662
Level 1 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	3,011	5,878
Level 2
ASSETS
Cash equivalents	0
Deposits with clearing organizations	0
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		169,569
Securities owned
Securities owned, at fair value	161,366
Securities purchased under agreements to resell		24,006
Derivative contracts
Derivative contracts, total		814
Total	162,082	194,389
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	15,668	14,699
Derivative contracts:
Derivative contracts	614	1,212
Total	16,282	15,911
Level 2 | TBAs
Derivative contracts
Derivative contracts, total	716	814
Derivative contracts:
Derivative contracts	614	1,212
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	24,455
Level 2 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	6,894	32,391
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13	12
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	608	1,894
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,179
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	12,538	17,074
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,357	2,536
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	4,037	5,024
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10	31
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	89,618	56,706
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		516
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	23,216	56,480
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	10,109	11,604
Level 3
ASSETS
Cash equivalents	0
Deposits with clearing organizations	0
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		84,970
Securities owned
Securities owned, at fair value	87,433
Derivative contracts
Derivative contracts, total		849
Total	87,602	85,977
Derivative contracts:
Derivative contracts	8	645
Total	8	645
Level 3 | ARS purchase commitments
Derivative contracts
Derivative contracts, total		849
Derivative contracts:
Derivative contracts	8	645
Level 3 | Investments
Securities owned
Investments	169	158	[1]
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	87,398	84,926
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	$ 35	44
Held for Sale Assets
Securities owned
Securities owned, at fair value		$ 3,600

[1] $3.6 million is included in other assets on the consolidated balance sheet.