Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
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	$ 999,627	$ 926,597
Securities Sold	147,886	94,486
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	999,627	926,597
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	21,659	12,538
Securities Sold	5,048	4,357
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	91,523	89,653
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	31,465	23,216
Securities Sold	$ 8,237	$ 10,109