Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	$ 735,393	$ 843,155
Securities sold	126,493	92,510
U.S. Government, agency and sovereign obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	509,614	570,607
Securities sold	77,485	30,615
Corporate debt and other obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	16,138	19,795
Securities sold	1,652	2,646
Mortgage and other asset-backed securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	3,504	6,689
Securities sold	27	255
Municipal obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	30,132	60,833
Securities sold	0	51
Convertible bonds
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	54,693	49,813
Securities sold	5,951	11,369
Equity securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	34,475	42,751
Securities sold	41,378	47,574
Money markets
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	35	1,245
Securities sold	0	0
Auction rate securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned	86,802	91,422
Securities sold	$ 0	$ 0