Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
ASSETS
Cash equivalents	$ 10,500	$ 10,500
Deposits with clearing organizations	30,340	34,599
Securities owned
Securities owned, at fair value	911,468	837,584
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	911,468	837,584
Investments	6,412
Derivative contracts
Total	952,412	887,657
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	137,286	85,446
Derivative contracts:
Derivative contracts	2,122	6,780
Total	139,408	92,226
Investments
Securities owned
Investments	104	101
Equity Securities [Member]
Securities owned
Securities owned, at fair value	27,795	28,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,974	14,774
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	3,150	7
Auction rate securities
Securities owned
Securities owned, at fair value	37,953	37,952
U.S. Treasury securities
Securities owned
Securities owned, at fair value	747,130	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	108,527	53,646
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	11,210	7,306
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3	3
Sovereign obligations
Securities owned
Securities owned, at fair value	305	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	240	78
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	7,921	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,324	7,236
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	8,905	2,486
Municipal obligations
Securities owned
Securities owned, at fair value	45,193	52,261
Convertible bonds
Securities owned
Securities owned, at fair value	21,906	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,218	9,709
Level 1
ASSETS
Cash equivalents	10,500	10,500
Deposits with clearing organizations	30,340	34,599
Securities owned
Securities owned, at fair value	779,062	686,242
Derivative contracts
Total	819,902	731,341
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	122,501	68,420
Derivative contracts:
Derivative contracts	961	811
Total	123,462	69,231
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	27,795	28,215
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	13,974	14,774
Level 1 | Money markets
Securities owned
Securities owned, at fair value	3,150	7
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	747,130	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	108,527	53,646
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	987	812
Level 2
Securities owned
Securities owned, at fair value	110,706	129,643
Derivative contracts
Total	110,706	134,516
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	14,785	17,026
Derivative contracts:
Derivative contracts	1,161	5,969
Total	15,946	22,995
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	16,253
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	10,223	6,494
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	3	3
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	305	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	240	78
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	7,921	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,324	7,236
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	8,905	2,486
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	45,193	52,261
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	21,906	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	9,218	9,709
Level 3
Securities owned
Securities owned, at fair value	21,700	21,699
Derivative contracts
Total	21,804	21,800
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	0
Derivative contracts:
Derivative contracts	0	0
Total	0	0
Level 3 | Investments
Securities owned
Investments	104	101
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	21,700	21,699
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		0
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	1,114	1,096
Auction Rate Securities Purchase Commitment | Level 2
Derivative contracts:
Derivative contracts	1,114	1,096
Auction Rate Securities Purchase Commitment | Level 3
Derivative contracts:
Derivative contracts	0	0
TBAs
Derivative contracts
Derivative contracts, total		4,873
Derivative contracts:
Derivative contracts	47	4,873
TBAs | Level 2
Derivative contracts
Derivative contracts, total		4,873
Derivative contracts:
Derivative contracts	47	4,873
Future [Member]
Derivative contracts:
Derivative contracts	959	807
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	959	807
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	2	4
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts	$ 2	4
Foreign exchange forward contracts | Level 2
Derivative contracts:
Derivative contracts		$ 0