Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
ASSETS
Cash equivalents	$ 21,831,000	$ 16,242,000
Deposits with clearing organizations	27,459,000	26,437,000
Securities owned
Securities owned, at fair value	1,002,705,000	707,108,000
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	1,003,267,000	710,668,000
Investments	7,355,000
Securities purchased under agreements to resell	3,896,000	24,006,000
Derivative contracts
Derivative contracts, total	1,427,000	1,663,000
Total	1,058,044,000	779,174,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	299,472,000	85,050,000
Derivative contracts:
Derivative contracts	1,874,000	2,024,000
Total	301,346,000	87,074,000
Futures
Derivative contracts:
Derivative contracts	726,000
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	4,000	1,000
Eurodollar Future [Member]
Derivative contracts:
TBAs		1,212,000
TBAs
Derivative contracts
Derivative contracts, total	549,000	814,000
Derivative contracts:
Derivative contracts		166,000
TBAs	785,000
ARS purchase commitments
Derivative contracts
Derivative contracts, total	878,000
Derivative contracts:
Derivative contracts	359,000	645,000
Investments
Securities owned
Investments	164,000	158,000
Corporate equities
Securities owned
Securities owned, at fair value	44,330,000	31,174,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	39,372,000	41,689,000
Money markets
Securities owned
Securities owned, at fair value	651,000	189,000
Auction rate securities
Securities owned
Securities owned, at fair value	89,743,000	84,926,000
U.S. Treasury securities (1)
Securities owned
Securities owned, at fair value	660,164,000	418,888,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	234,072,000	28,662,000
U.S. Agency securities
Securities owned
Securities owned, at fair value	44,885,000	38,269,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,000	12,000
Sovereign obligations
Securities owned
Securities owned, at fair value	1,451,000	1,894,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,108,000
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	33,078,000	17,074,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,059,000	2,536,000
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,387,000	5,024,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	156,000	31,000
Municipal obligations
Securities owned
Securities owned, at fair value	81,730,000	56,750,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		516,000
Convertible bonds
Securities owned
Securities owned, at fair value	44,848,000	56,480,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,700,000	11,604,000
Level 1
ASSETS
Cash equivalents	21,831,000	16,242,000
Deposits with clearing organizations	27,459,000	26,437,000
Securities owned
Securities owned, at fair value	726,234,000	456,129,000
Derivative contracts
Total	775,524,000	498,808,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	273,444,000	70,351,000
Derivative contracts:
Derivative contracts	730,000	167,000
Total	274,174,000	70,518,000
Level 1 | Futures
Derivative contracts:
Derivative contracts	726,000
Level 1 | Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	4,000	1,000
Level 1 | TBAs
Derivative contracts:
Derivative contracts		166,000
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	44,330,000	31,174,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	39,372,000	41,689,000
Level 1 | Money markets
Securities owned
Securities owned, at fair value	651,000	189,000
Level 1 | U.S. Treasury securities (1)
Securities owned
Securities owned, at fair value	660,164,000	418,888,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	234,072,000	28,662,000
Level 1 | U.S. Agency securities
Securities owned
Securities owned, at fair value	21,089,000	5,878,000
Level 2
ASSETS
Cash equivalents	0
Deposits with clearing organizations	0
Securities owned
Securities owned, at fair value	187,254,000	169,569,000
Securities purchased under agreements to resell	3,896,000	24,006,000
Derivative contracts
Derivative contracts, total	549,000	814,000
Total	191,699,000	194,389,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	26,028,000	14,699,000
Derivative contracts:
Derivative contracts	785,000	1,212,000
Total	26,813,000	15,911,000
Level 2 | Eurodollar Future [Member]
Derivative contracts:
TBAs		1,212,000
Level 2 | TBAs
Derivative contracts
Derivative contracts, total	549,000	814,000
Derivative contracts:
TBAs	785,000
Level 2 | U.S. Agency securities
Securities owned
Securities owned, at fair value	23,796,000	32,391,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	5,000	12,000
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	1,451,000	1,894,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	1,108,000
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	33,078,000	17,074,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	17,059,000	2,536,000
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	2,387,000	5,024,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	156,000	31,000
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	81,694,000	56,706,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value		516,000
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	44,848,000	56,480,000
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	7,700,000	11,604,000
Level 3
ASSETS
Cash equivalents	0
Securities owned
Securities owned, at fair value	89,779,000	84,970,000
Derivative contracts
Derivative contracts, total	878,000	849,000
Total	90,821,000	85,977,000
Derivative contracts:
Derivative contracts	359,000	645,000
Total	359,000	645,000
Level 3 | ARS purchase commitments
Derivative contracts
Derivative contracts, total	878,000
Derivative contracts:
Derivative contracts	359,000	645,000
Level 3 | Investments
Securities owned
Investments	164,000	158,000
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	89,743,000	84,926,000
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	36,000	44,000
Held for Sale Assets [Member]
Securities owned
Securities owned, at fair value	$ 562,000	$ 3,600,000