Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
ASSETS
Cash equivalents	$ 28,075	$ 31,175
Deposits with clearing organizations	28,194	24,188
Securities owned:
Securities Owned	955,363	843,155
Investments	8,465us-gaap_InvestmentsFairValueDisclosure
Loans held for sale	95,876	19,243
Securities purchased under agreements to resell		250,000
Derivative contracts:
Derivative contracts, total	16,782us-gaap_DerivativeAssets	12,111us-gaap_DerivativeAssets
Total	1,185,927us-gaap_AssetsFairValueDisclosure	1,240,626us-gaap_AssetsFairValueDisclosure
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	184,649	92,510
Derivative contracts:
Derivative contracts	2,025us-gaap_DerivativeLiabilities	3,505us-gaap_DerivativeLiabilities
Total	186,674us-gaap_LiabilitiesFairValueDisclosure	96,015us-gaap_LiabilitiesFairValueDisclosure
Future [Member]
Derivative contracts:
Derivative contracts	482	353
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	3	10
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts, total	5,358	4,535
Derivative contracts:
TBAs	199	1,018
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	11,424	7,576
Derivative contracts:
Derivative contracts	544	1,222
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	797	902
Corporate Equities [Member]
Securities owned:
Securities Owned	49,760	42,751
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	37,023	47,574
Money Markets [Member]
Securities owned:
Securities Owned	391	1,245
Auction Rate Securities [Member]
Securities owned:
Securities Owned	99,057	91,422
Investments Included in Other Assets [Member]
Securities owned:
Investments	61,637	60,754
U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	641,665	540,223
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	128,927	30,581
U.S. Agency Securities [Member]
Securities owned:
Securities Owned	19,927	26,261
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	22	34
Sovereign Obligations [Member]
Securities owned:
Securities Owned	772	4,123
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	14,813	19,795
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	4,533	2,646
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	3,982	6,689
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value		255
Municipal Obligations [Member]
Securities owned:
Securities Owned	83,942	60,833
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	51	51
Convertible Bonds [Member]
Securities owned:
Securities Owned	41,054	49,813
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	11,453	11,369
Level 1 [Member]
ASSETS
Cash equivalents	28,075	31,175
Deposits with clearing organizations	28,194	24,188
Securities owned:
Securities Owned	694,735	584,219
Derivative contracts:
Total	751,004	639,582
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	165,950	78,155
Derivative contracts:
Derivative contracts	485	363
Total	166,435	78,518
Level 1 [Member] \| Future [Member]
Derivative contracts:
Derivative contracts	482	353
Level 1 [Member] \| Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	3	10
Level 1 [Member] \| Corporate Equities [Member]
Securities owned:
Securities Owned	49,760	42,751
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	37,023	47,574
Level 1 [Member] \| Money Markets [Member]
Securities owned:
Securities Owned	391	1,245
Level 1 [Member] \| Investments Included in Other Assets [Member]
Securities owned:
Investments	0	0
Level 1 [Member] \| U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	641,665	540,223
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	128,927	30,581
Level 2 [Member]
Securities owned:
Securities Owned	161,467	167,350
Loans held for sale	95,876	19,243
Securities purchased under agreements to resell		250,000
Derivative contracts:
Derivative contracts, total	5,358	4,535
Total	315,189	492,374
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	18,699	14,355
Derivative contracts:
Derivative contracts	199	1,018
Total	18,898	15,373
Level 2 [Member] \| To Be Announced Security [Member]
Derivative contracts:
Derivative contracts, total	5,358	4,535
Derivative contracts:
TBAs	199	1,018
Level 2 [Member] \| Investments Included in Other Assets [Member]
Securities owned:
Investments	52,488	51,246
Level 2 [Member] \| U.S. Agency Securities [Member]
Securities owned:
Securities Owned	17,008	26,261
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	22	34
Level 2 [Member] \| Sovereign Obligations [Member]
Securities owned:
Securities Owned	772	4,123
Level 2 [Member] \| Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	14,813	19,795
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	4,533	2,646
Level 2 [Member] \| Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	3,982	6,689
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value		255
Level 2 [Member] \| Municipal Obligations [Member]
Securities owned:
Securities Owned	83,838	60,669
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	51	51
Level 2 [Member] \| Convertible Bonds [Member]
Securities owned:
Securities Owned	41,054	49,813
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	11,453	11,369
Level 3 [Member]
Securities owned:
Securities Owned	99,161	91,586
Derivative contracts:
Derivative contracts, total	11,424	7,576
Total	119,734	108,670
Derivative contracts:
Derivative contracts	1,341	2,124
Total	1,341	2,124
Level 3 [Member] \| Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	11,424	7,576
Derivative contracts:
Derivative contracts	544	1,222
Level 3 [Member] \| Auction Rate Securities [Member]
Securities owned:
Securities Owned	99,057	91,422
Level 3 [Member] \| Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	797	902
Level 3 [Member] \| Investments Included in Other Assets [Member]
Securities owned:
Investments	9,149	9,508
Level 3 [Member] \| Municipal Obligations [Member]
Securities owned:
Securities Owned	$ 104	$ 164

Deposits with clearing organizations fair value.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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