Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	$ 843,155	$ 856,088
Securities Sold	92,510	76,314
U.S. Government, agency and sovereign obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	570,607	596,114
Securities Sold	30,615	11,889
Corporate debt and other obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	19,795	14,673
Securities Sold	2,646	4,847
Mortgage and other asset-backed securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	6,689	3,395
Securities Sold	255	7
Municipal obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	60,833	40,166
Securities Sold	51	72
Convertible bonds
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	49,813	53,719
Securities Sold	11,369	13,922
Corporate equities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	42,751	61,634
Securities Sold	47,574	45,336
Money markets
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	1,245	1,263
Securities Sold	0	241
Auction rate securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities Owned	91,422	85,124
Securities Sold	$ 0	$ 0