Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 1,005,150	$ 843,155
Securities Sold	220,667	92,510
US Treasury and Government [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	701,009	570,607
Securities Sold	159,594	30,615
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	23,623	19,795
Securities Sold	6,045	2,646
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	4,718	6,689
Securities Sold	0	255
Municipal Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	66,030	60,833
Securities Sold	49	51
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	61,711	49,813
Securities Sold	8,194	11,369
Corporate Equities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	46,933	42,751
Securities Sold	46,785	47,574
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	742	1,245
Securities Sold	0	0
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	100,384	91,422
Securities Sold	$ 0	$ 0