Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 1,036,782	$ 735,393
Securities Sold	270,952	126,493
US Treasury and Government [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	749,323	509,614
Securities Sold	204,003	77,485
Corporate Debt and Other Obligations [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	22,823	16,138
Securities Sold	11,166	1,652
Mortgage and Other Asset-Backed Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	4,151	3,504
Securities Sold	27	27
Municipal Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	80,220	30,132
Securities Sold	0	0
Convertible Bonds [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	50,984	54,693
Securities Sold	15,988	5,951
Equity Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	44,964	34,475
Securities Sold	39,768	41,378
Money Markets [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	132	35
Securities Sold	0	0
Auction Rate Securities [Member]
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	84,185	86,802
Securities Sold	$ 0	$ 0