Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
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	$ 995,370	$ 837,584
Securities Sold	155,979	85,446
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	995,370	837,584
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	25,556	20,665
Securities Sold	20,907	7,236
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	141,362	52,261
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	26,123	31,270
Securities Sold	$ 17,779	$ 9,709