Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
ASSETS
Cash equivalents		$ 10,500
Deposits with clearing organizations	$ 36,461	34,599
Securities owned
Securities owned, at fair value	653,194	837,584
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	653,194	837,584
Investments	5,912
Derivative contracts
Total	689,667	887,657
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	121,880	85,446
Derivative contracts:
Derivative contracts	1,337	6,780
Total	123,217	92,226
Investments
Securities owned
Investments		101
Equity Securities [Member]
Securities owned
Securities owned, at fair value	31,822	28,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,450	14,774
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	363	7
Auction rate securities
Securities owned
Securities owned, at fair value	25,314	37,952
U.S. Treasury securities
Securities owned
Securities owned, at fair value	462,298	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	74,020	53,646
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	8,740	7,306
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7	3
Sovereign obligations
Securities owned
Securities owned, at fair value	2,571	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,503	78
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	24,448	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,683	7,236
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,292	2,486
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1
Municipal obligations
Securities owned
Securities owned, at fair value	66,550	52,261
Convertible bonds
Securities owned
Securities owned, at fair value	28,796	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	16,216	9,709
Level 1
ASSETS
Cash equivalents		10,500
Deposits with clearing organizations	36,461	34,599
Securities owned
Securities owned, at fair value	496,088	686,242
Derivative contracts
Total	532,549	731,341
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	91,470	68,420
Derivative contracts:
Derivative contracts	322	811
Total	91,792	69,231
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	31,822	28,215
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,450	14,774
Level 1 | Money markets
Securities owned
Securities owned, at fair value	363	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	462,298	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	74,020	53,646
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	1,605	812
Level 2
Securities owned
Securities owned, at fair value	157,106	129,643
Derivative contracts
Total	157,118	134,516
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	30,410	17,026
Derivative contracts:
Derivative contracts	1,015	5,969
Total	31,425	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	25,314	16,253
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	7,135	6,494
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7	3
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	2,571	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,503	78
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	24,448	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,683	7,236
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,292	2,486
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	66,550	52,261
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	28,796	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	16,216	9,709
Level 3
Securities owned
Securities owned, at fair value	0	21,699
Derivative contracts
Total	0	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		101
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	0	21,699
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value		0
Auction Rate Securities Purchase Commitment
Derivative contracts:
Derivative contracts	1,006	1,096
Auction Rate Securities Purchase Commitment | Level 2
Derivative contracts:
Derivative contracts	1,006	1,096
Auction Rate Securities Purchase Commitment | Level 3
Derivative contracts:
Derivative contracts	0	0
TBAs
Derivative contracts
Derivative contracts, total	12	4,873
Derivative contracts:
Derivative contracts	9	4,873
TBAs | Level 2
Derivative contracts
Derivative contracts, total	12	4,873
Derivative contracts:
Derivative contracts	9	4,873
Future [Member]
Derivative contracts:
Derivative contracts	322	807
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	$ 322	807
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts		4
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts		4
Foreign exchange forward contracts | Level 2
Derivative contracts:
Derivative contracts		$ 0