Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015
ASSETS
Cash equivalents	$ 16,242		$ 13,000
Deposits with clearing organizations	26,437		31,456
Securities owned
Securities owned, at fair value	707,108		734,831
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	710,668		735,393
Investments	6,848
Loans held for sale			60,234
Securities purchased under agreements to resell	24,006	[1]	206,499	[2]
Derivative contracts
Derivative contracts, total	1,663		15,609
Total	779,174		1,062,348
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	85,050		126,493
Derivative contracts:
Derivative contracts	2,024		14,162
Total	87,074		140,655
Futures
Derivative contracts:
Derivative contracts	166
Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	1		2
Eurodollar Future [Member]
Derivative contracts:
TBAs			11,619
TBAs
Derivative contracts
Derivative contracts, total	814		6,448
Derivative contracts:
Derivative contracts			249
TBAs	1,212
Interest Rate Lock Commitments [Member]
Derivative contracts
Derivative contracts, total			9,161
Derivative contracts:
Derivative contracts			923
ARS purchase commitments
Derivative contracts
Derivative contracts, total	849
Derivative contracts:
Derivative contracts	645		1,369
Investments
Securities owned
Investments	158	[3]	157	[4]
Corporate equities
Securities owned
Securities owned, at fair value	31,174		34,475
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	41,689		41,378
Money markets
Securities owned
Securities owned, at fair value	189		35
Auction rate securities
Securities owned
Securities owned, at fair value	84,926		86,802
U.S. Treasury securities (1)
Securities owned
Securities owned, at fair value	418,888		436,533
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	28,662		75,653
U.S. Agency securities
Securities owned
Securities owned, at fair value	38,269		71,416
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	12		15
Sovereign obligations
Securities owned
Securities owned, at fair value	1,894		1,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value			1,817
Corporate debt and other obligations
Securities owned
Securities owned, at fair value	17,074		16,138
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,536		1,652
Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	5,024		3,504
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	31		27
Municipal obligations
Securities owned
Securities owned, at fair value	56,750		30,132
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	516		0
Convertible bonds
Securities owned
Securities owned, at fair value	56,480		54,693
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,604		5,951
Level 1
ASSETS
Cash equivalents	16,242		13,000
Deposits with clearing organizations	26,437		31,456
Securities owned
Securities owned, at fair value	456,129		496,283
Derivative contracts
Total	498,808		540,739
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	70,351		117,031
Derivative contracts:
Derivative contracts	167		251
Total	70,518		117,282
Level 1 | Futures
Derivative contracts:
Derivative contracts	166
Level 1 | Foreign exchange forward contracts
Derivative contracts:
Derivative contracts	1		2
Level 1 | TBAs
Derivative contracts:
Derivative contracts			249
Level 1 | Corporate equities
Securities owned
Securities owned, at fair value	31,174		34,475
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	41,689		41,378
Level 1 | Money markets
Securities owned
Securities owned, at fair value	189		35
Level 1 | U.S. Treasury securities (1)
Securities owned
Securities owned, at fair value	418,888		436,533
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	28,662		75,653
Level 1 | U.S. Agency securities
Securities owned
Securities owned, at fair value	5,878		25,240
Level 2
ASSETS
Cash equivalents	0
Deposits with clearing organizations	0
Securities owned
Securities owned, at fair value	169,569		152,227
Loans held for sale			60,234
Securities purchased under agreements to resell	24,006	[1]	206,499	[2]
Derivative contracts
Derivative contracts, total	814		6,448
Total	194,389		425,408
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	14,699		9,462
Derivative contracts:
Derivative contracts	1,212		11,619
Total	15,911		21,081
Level 2 | Eurodollar Future [Member]
Derivative contracts:
TBAs			11,619
Level 2 | TBAs
Derivative contracts
Derivative contracts, total	814		6,448
Derivative contracts:
TBAs	1,212
Level 2 | U.S. Agency securities
Securities owned
Securities owned, at fair value	32,391		46,176
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	12		15
Level 2 | Sovereign obligations
Securities owned
Securities owned, at fair value	1,894		1,665
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value			1,817
Level 2 | Corporate debt and other obligations
Securities owned
Securities owned, at fair value	17,074		16,138
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	2,536		1,652
Level 2 | Mortgage and other asset-backed securities
Securities owned
Securities owned, at fair value	5,024		3,504
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	31		27
Level 2 | Municipal obligations
Securities owned
Securities owned, at fair value	56,706		30,051
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	516
Level 2 | Convertible bonds
Securities owned
Securities owned, at fair value	56,480		54,693
Securities sold, but not yet purchased
Securities sold but not yet purchased, at fair value	11,604		5,951
Level 3
ASSETS
Cash equivalents	0
Securities owned
Securities owned, at fair value	84,970		86,883
Derivative contracts
Derivative contracts, total	849		9,161
Total	85,977		96,201
Derivative contracts:
Derivative contracts	645		2,292
Total	645		2,292
Level 3 | Interest Rate Lock Commitments [Member]
Derivative contracts
Derivative contracts, total			9,161
Derivative contracts:
Derivative contracts			923
Level 3 | ARS purchase commitments
Derivative contracts
Derivative contracts, total	849
Derivative contracts:
Derivative contracts	645		1,369
Level 3 | Investments
Securities owned
Investments	158	[3]	157	[4]
Level 3 | Auction rate securities
Securities owned
Securities owned, at fair value	84,926		86,802
Level 3 | Municipal obligations
Securities owned
Securities owned, at fair value	$ 44		$ 81

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