Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 895,251	$ 926,597
Securities Sold	143,746	94,486
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	895,251	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	14,400	12,538
Securities Sold	4,061	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	45,427	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	34,013	23,216
Securities Sold	$ 7,356	$ 10,109