Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
ASSETS
Cash equivalents	$ 566	$ 31,175
Deposits with clearing organizations	28,190	24,188
Securities owned:
Securities Owned	1,005,150	843,155
Investments	8,405
Loans held for sale	76,351	19,243
Securities purchased under agreements to resell	104,301	250,000
Derivative contracts:
Derivative contracts, total	7,646	12,111
Total	1,286,128	1,240,626
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	220,667	92,510
Derivative contracts:
Derivative contracts	2,614	3,505
Total	223,281	96,015
Future [Member]
Derivative contracts:
Derivative contracts	698	353
Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	28	10
To Be Announced Security [Member]
Derivative contracts:
Derivative contracts, total	2,586	4,535
Derivative contracts:
TBAs	171	1,018
Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	5,060	7,576
Derivative contracts:
Derivative contracts	683	1,222
Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,034	902
Corporate Equities [Member]
Securities owned:
Securities Owned	46,933	42,751
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	46,785	47,574
Money Markets [Member]
Securities owned:
Securities Owned	742	1,245
Auction Rate Securities [Member]
Securities owned:
Securities Owned	100,384	91,422
Investments Included in Other Assets [Member]
Securities owned:
Investments	63,924	60,754
U.S. Treasury Securities [Member]
Securities owned:
Securities Owned	677,766	540,223
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	159,576	30,581
U.S. Agency Securities [Member]
Securities owned:
Securities Owned	22,647	26,261
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	18	34
Sovereign Obligations [Member]
Securities owned:
Securities Owned	596	4,123
Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	23,623	19,795
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	6,045	2,646
Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	4,718	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	66,030	60,833
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	49	51
Convertible Bonds [Member]
Securities owned:
Securities Owned	61,711	49,813
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	8,194	11,369
Level 1 [Member]
ASSETS
Cash equivalents	566	31,175
Deposits with clearing organizations	28,190	24,188
Securities owned:
Securities Owned	726,192	584,219
Derivative contracts:
Total	754,948	639,582
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	206,361	78,155
Derivative contracts:
Derivative contracts	726	363
Total	207,087	78,518
Level 1 [Member] | Future [Member]
Derivative contracts:
Derivative contracts	698	353
Level 1 [Member] | Foreign Exchange Forward [Member]
Derivative contracts:
Derivative contracts	28	10
Level 1 [Member] | Corporate Equities [Member]
Securities owned:
Securities Owned	46,933	42,751
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	46,785	47,574
Level 1 [Member] | Money Markets [Member]
Securities owned:
Securities Owned	742	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	677,766	540,223
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	159,576	30,581
Level 1 [Member] | U.S. Agency Securities [Member]
Securities owned:
Securities Owned	751
Level 2 [Member]
Securities owned:
Securities Owned	178,512	167,350
Loans held for sale	76,351	19,243
Securities purchased under agreements to resell	104,301	250,000
Derivative contracts:
Derivative contracts, total	2,586	4,535
Total	416,560	492,374
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	14,306	14,355
Derivative contracts:
Derivative contracts	171	1,018
Total	14,477	15,373
Level 2 [Member] | To Be Announced Security [Member]
Derivative contracts:
Derivative contracts, total	2,586	4,535
Derivative contracts:
TBAs	171	1,018
Level 2 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	54,810	51,246
Level 2 [Member] | U.S. Agency Securities [Member]
Securities owned:
Securities Owned	21,896	26,261
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	18	34
Level 2 [Member] | Sovereign Obligations [Member]
Securities owned:
Securities Owned	596	4,123
Level 2 [Member] | Corporate Debt and Other Obligations [Member]
Securities owned:
Securities Owned	23,623	19,795
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	6,045	2,646
Level 2 [Member] | Mortgage and Other Asset-Backed Securities [Member]
Securities owned:
Securities Owned	4,718	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	65,968	60,669
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	49	51
Level 2 [Member] | Convertible Bonds [Member]
Securities owned:
Securities Owned	61,711	49,813
Financial Instruments Sold, Not yet Purchased, at Fair Value [Abstract]
Securities sold, but not yet purchased, at fair value	8,194	11,369
Level 3 [Member]
Securities owned:
Securities Owned	100,446	91,586
Derivative contracts:
Derivative contracts, total	5,060	7,576
Total	114,620	108,670
Derivative contracts:
Derivative contracts	1,717	2,124
Total	1,717	2,124
Level 3 [Member] | Interest Rate Lock Commitments [Member]
Derivative contracts:
Derivative contracts, total	5,060	7,576
Derivative contracts:
Derivative contracts	683	1,222
Level 3 [Member] | Auction Rate Securities [Member]
Securities owned:
Securities Owned	100,384	91,422
Level 3 [Member] | Auction Rate Securities Purchase Commitment [Member]
Derivative contracts:
Derivative contracts	1,034	902
Level 3 [Member] | Investments Included in Other Assets [Member]
Securities owned:
Investments	9,114	9,508
Level 3 [Member] | Municipal Obligations [Member]
Securities owned:
Securities Owned	$ 62	$ 164