Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
ASSETS
Cash equivalents	$ 10,500	$ 10,490
Deposits with clearing organizations	34,599	34,293
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		926,597
Securities owned
Securities owned, at fair value	837,584	926,597
Investments	6,504
Derivative contracts
Total	887,657	972,265
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	85,446	94,486
Derivative contracts:
Derivative contracts	6,780	1,388
Total	92,226	95,874
Investments
Securities owned
Investments	101	169
Equity Securities [Member]
Securities owned
Securities owned, at fair value	28,215	34,067
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	14,774
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	7	383
Auction rate securities
Securities owned
Securities owned, at fair value	37,952	111,853
Corporate Equities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		25,393
U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		640,337
Securities owned
Securities owned, at fair value	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	53,646	53,425
US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	7,306	9,905
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		13
US Government Agencies Debt Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3
Sovereign obligations
Securities owned
Securities owned, at fair value	214	608
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	78	1,179
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	20,665	12,538
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,236	4,357
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,486	4,037
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		10
Municipal obligations
Securities owned
Securities owned, at fair value	52,261	89,653
Convertible bonds
Securities owned
Securities owned, at fair value	31,270	23,216
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,709	10,109
Level 1
ASSETS
Cash equivalents	10,500	10,490
Deposits with clearing organizations	34,599	34,293
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		677,798
Securities owned
Securities owned, at fair value	686,242
Derivative contracts
Total	731,341	722,581
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	68,420	78,818
Derivative contracts:
Derivative contracts	811	766
Total	69,231	79,584
Level 1 | Equity Securities [Member]
Securities owned
Securities owned, at fair value	28,215	34,067
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	14,774
Level 1 | Money Market Funds [Member]
Securities owned
Securities owned, at fair value	7	383
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		25,393
Level 1 | U.S. Treasury securities
ASSETS
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned		640,337
Securities owned
Securities owned, at fair value	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	53,646	53,425
Level 1 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	812	3,011
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		161,366
Securities owned
Securities owned, at fair value	129,643
Derivative contracts
Total	134,516	162,082
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,026	15,668
Derivative contracts:
Derivative contracts	5,969	614
Total	22,995	16,282
Level 2 | Auction rate securities
Securities owned
Securities owned, at fair value	16,253	24,455
Level 2 | US Government Corporations and Agencies Securities [Member]
Securities owned
Securities owned, at fair value	6,494	6,894
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3	13
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	214	608
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	78	1,179
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	20,665	12,538
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,236	4,357
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,486	4,037
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		10
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	52,261	89,618
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	31,270	23,216
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,709	10,109
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		87,433
Securities owned
Securities owned, at fair value	21,699
Derivative contracts
Total	21,800	87,602
Derivative contracts:
Derivative contracts	0	8
Total	0	8
Level 3 | Investments
Securities owned
Investments	101	169
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	21,699	87,398
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	0	35
TBAs
Derivative contracts
Derivative contracts, total	4,873	716
Derivative contracts:
Derivative contracts	4,873	614
TBAs | Level 2
Derivative contracts
Derivative contracts, total	4,873	716
Derivative contracts:
Derivative contracts	4,873	614
ARS purchase commitments
Derivative contracts:
Derivative contracts	1,096	8
ARS purchase commitments | Level 2
Derivative contracts:
Derivative contracts	1,096
ARS purchase commitments | Level 3
Derivative contracts:
Derivative contracts	0	8
Future [Member]
Derivative contracts:
Derivative contracts	807	766
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	807	$ 766
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	4
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts	$ 4