Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
ASSETS
Cash equivalents	$ 10,500	$ 10,490
Deposits with clearing organizations	31,291	34,293
Securities owned
Securities owned, at fair value	895,251	926,597
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	895,251	926,597
Investments	6,605
Derivative contracts
Total	949,455	972,265
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	143,746	94,486
Derivative contracts:
Derivative contracts	14,048	1,388
Total	157,794	95,874
Investments
Securities owned
Investments	104	169
Equity Securities [Member]
Securities owned
Securities owned, at fair value	34,613	34,067
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	22,947	25,393
Money Market Funds [Member]
Securities owned
Securities owned, at fair value		383
Auction rate securities
Securities owned
Securities owned, at fair value	37,683	111,853
U.S. Treasury securities
Securities owned
Securities owned, at fair value	701,261	640,337
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	107,031	53,425
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	22,113	9,905
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2	13
Sovereign obligations
Securities owned
Securities owned, at fair value	2,964	608
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	14,400	12,538
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,061	4,357
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,777	4,037
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,349	10
Municipal obligations
Securities owned
Securities owned, at fair value	45,427	89,653
Convertible bonds
Securities owned
Securities owned, at fair value	34,013	23,216
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,356	10,109
Level 1
ASSETS
Cash equivalents	10,500	10,490
Deposits with clearing organizations	31,291	34,293
Securities owned
Securities owned, at fair value	750,562	677,798
Derivative contracts
Total	792,353	722,581
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	129,978	78,818
Derivative contracts:
Derivative contracts	711	766
Total	130,689	79,584
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	34,613	34,067
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	22,947	25,393
Level 1 | Money markets
Securities owned
Securities owned, at fair value		383
Level 1 | Auction rate securities
Securities owned
Securities owned, at fair value	0
Level 1 | U.S. Treasury securities
Securities owned
Securities owned, at fair value	701,261	640,337
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	107,031	53,425
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	14,688	3,011
Level 2
Securities owned
Securities owned, at fair value	123,259	161,366
Derivative contracts
Total	135,568	162,082
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,768	15,668
Derivative contracts:
Derivative contracts	13,336	614
Total	27,104	16,282
Level 2 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	16,253	24,455
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	7,425	6,894
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2	13
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	2,964	608
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	14,400	12,538
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	4,061	4,357
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,777	4,037
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,349	10
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	45,427	89,618
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	34,013	23,216
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,356	10,109
Level 3
Securities owned
Securities owned, at fair value	21,430	87,433
Derivative contracts
Total	21,534	87,602
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	1	8
Total	1	8
Level 3 | Investments
Securities owned
Investments	104	169
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	21,430	87,398
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		35
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	1,079	8
Auction Rate Securities Purchase Commitment | Level 2
Derivative contracts:
Derivative contracts	1,078
Auction Rate Securities Purchase Commitment | Level 3
Derivative contracts:
Derivative contracts	1	8
TBAs
Derivative contracts
Derivative contracts, total	12,309	716
Derivative contracts:
Derivative contracts	12,258	614
TBAs | Level 2
Derivative contracts
Derivative contracts, total	12,309	716
Derivative contracts:
Derivative contracts	12,258	614
Future [Member]
Derivative contracts:
Derivative contracts	709	766
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	709	$ 766
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	2
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts	$ 2