Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
ASSETS
Cash equivalents	$ 10,490	$ 10,490
Deposits with clearing organizations	41,387	34,293
Securities owned
Securities owned, at fair value	999,627	926,597
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	999,627	926,597
Investments	7,536
Derivative contracts
Total	1,055,424	972,265
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	147,886	94,486
Derivative contracts:
Derivative contracts	5,068	1,388
Total	152,954	95,874
Future [Member]
Derivative contracts:
Derivative contracts	1,218	766
TBAs
Derivative contracts
Derivative contracts, total	3,752	716
Derivative contracts:
Derivative contracts	3,667	614
ARS purchase commitments
Derivative contracts:
Derivative contracts	183	8
Investments
Securities owned
Investments	168	169
Equity Securities [Member]
Securities owned
Securities owned, at fair value	31,974	34,067
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	24,433	25,393
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	573	383
Auction rate securities
Securities owned
Securities owned, at fair value	87,350	111,853
U.S. Treasury securities
Securities owned
Securities owned, at fair value	708,250	640,337
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	108,819	53,425
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	24,296	9,905
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5	13
Sovereign obligations
Securities owned
Securities owned, at fair value	557	608
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,336	1,179
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	21,659	12,538
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,048	4,357
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	1,980	4,037
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	8	10
Municipal obligations
Securities owned
Securities owned, at fair value	91,523	89,653
Convertible bonds
Securities owned
Securities owned, at fair value	31,465	23,216
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	8,237	10,109
Level 1
ASSETS
Cash equivalents	10,490	10,490
Deposits with clearing organizations	41,387	34,293
Securities owned
Securities owned, at fair value	754,353	677,798
Derivative contracts
Total	806,230	722,581
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	108,819	78,818
Derivative contracts:
Derivative contracts	1,218	766
Total	110,037	79,584
Level 1 | Future [Member]
Derivative contracts:
Derivative contracts	1,218	766
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	31,974	34,067
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		25,393
Level 1 | Money markets
Securities owned
Securities owned, at fair value	573	383
Level 1 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	708,250	640,337
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	108,819	53,425
Level 1 | U.S. Agency securities
Securities owned
Securities owned, at fair value	13,556	3,011
Level 2
Securities owned
Securities owned, at fair value	157,924	161,366
Derivative contracts
Total	161,676	162,082
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	39,067	15,668
Derivative contracts:
Derivative contracts	3,667	614
Total	42,734	16,282
Level 2 | TBAs
Derivative contracts
Derivative contracts, total	3,752	716
Derivative contracts:
Derivative contracts	3,667	614
Level 2 | Corporate equities
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	24,433
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value		24,455
Level 2 | U.S. Agency securities
Securities owned
Securities owned, at fair value	10,740	6,894
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5	13
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	557	608
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,336	1,179
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	21,659	12,538
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,048	4,357
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	1,980	4,037
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	8	10
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	91,523	89,618
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	31,465	23,216
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	8,237	10,109
Level 3
Securities owned
Securities owned, at fair value	87,350	87,433
Derivative contracts
Total	87,518	87,602
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	183	8
Total	183	8
Level 3 | ARS purchase commitments
Derivative contracts:
Derivative contracts	183	8
Level 3 | Investments
Securities owned
Investments	168	169
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	87,350	87,398
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	$ 0	$ 35