Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
ASSETS
Cash equivalents	$ 10,500	$ 10,500
Deposits with clearing organizations	31,293	34,599
Securities owned
Securities owned, at fair value	995,370	837,584
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	995,370	837,584
Investments	6,329
Derivative contracts
Total	1,037,283	887,657
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	155,979	85,446
Derivative contracts:
Derivative contracts	2,594	6,780
Total	158,573	92,226
Investments
Securities owned
Investments	106	101
Equity Securities [Member]
Securities owned
Securities owned, at fair value	39,430	28,215
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,487	14,774
Money Market Funds [Member]
Securities owned
Securities owned, at fair value	2,674	7
Auction rate securities
Securities owned
Securities owned, at fair value	42,461	37,952
U.S. Treasury securities
Securities owned
Securities owned, at fair value	689,992	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	97,720	53,646
US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	24,753	7,306
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	19	3
Sovereign obligations
Securities owned
Securities owned, at fair value	1,063	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,066	78
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	25,556	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	20,907	7,236
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	1,956	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	141,362	52,261
Convertible bonds
Securities owned
Securities owned, at fair value	26,123	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,779	9,709
Level 1
ASSETS
Cash equivalents	10,500	10,500
Deposits with clearing organizations	31,293	34,599
Securities owned
Securities owned, at fair value	736,636	686,242
Derivative contracts
Total	778,429	731,341
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	116,207	68,420
Derivative contracts:
Derivative contracts	1,785	811
Total	117,992	69,231
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	39,430	28,215
Level 1 | Equity Securities [Member]
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	18,487	14,774
Level 1 | Money markets
Securities owned
Securities owned, at fair value	2,674	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	689,992	657,208
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	97,720	53,646
Level 1 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	4,540	812
Level 2
Securities owned
Securities owned, at fair value	258,734	129,643
Derivative contracts
Total	258,854	134,516
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	39,772	17,026
Derivative contracts:
Derivative contracts	809	5,969
Total	40,581	22,995
Level 2 | Investments
Securities owned
Investments	106
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	42,461	16,253
Level 2 | US Government Agencies Debt Securities [Member]
Securities owned
Securities owned, at fair value	20,213	6,494
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	19	3
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	1,063	214
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,066	78
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	25,556	20,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	20,907	7,236
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	1,956	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	141,362	52,261
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	26,123	31,270
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,779	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	764	1,096
Auction Rate Securities Purchase Commitment | Level 2
Derivative contracts:
Derivative contracts	764	1,096
Auction Rate Securities Purchase Commitment | Level 3
Derivative contracts:
Derivative contracts	0	0
TBAs
Derivative contracts
Derivative contracts, total	14	4,873
Derivative contracts:
Derivative contracts	45	4,873
TBAs | Level 2
Derivative contracts
Derivative contracts, total	14	4,873
Derivative contracts:
Derivative contracts	45	4,873
Future [Member]
Derivative contracts:
Derivative contracts	1,781	807
Future [Member] | Level 1
Derivative contracts:
Derivative contracts	1,781	807
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	4	4
Foreign exchange forward contracts | Level 1
Derivative contracts:
Derivative contracts	$ 4	4
Foreign exchange forward contracts | Level 2
Derivative contracts:
Derivative contracts		$ 0