Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	$ 707,108,000	$ 734,831,000
Securities Sold	85,050,000	126,493,000
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	710,668,000	735,393,000
U.S. treasury, agency and sovereign obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	459,051,000	509,614,000
Securities Sold	28,674,000	77,485,000
Corporate debt and other obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	17,074,000	16,138,000
Securities Sold	2,536,000	1,652,000
Mortgage and other asset-backed securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	5,024,000	3,504,000
Securities Sold	31,000	27,000
Municipal obligations
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	56,750,000	30,132,000
Securities Sold	516,000	0
Convertible bonds
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	56,480,000	54,693,000
Securities Sold	11,604,000	5,951,000
Corporate equities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	31,174,000	34,475,000
Securities Sold	41,689,000	41,378,000
Money markets
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	189,000	35,000
Securities Sold	0	0
Auction rate securities
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	84,926,000	86,802,000
Securities Sold	0	0
Held for Sale Assets
Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items]
Securities owned, including amounts pledged of $0 ($546,334 in 2015), at fair value	$ 3,600,000	$ 562,000