Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
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	$ 561,683	$ 610,517
Securities Sold	84,145	126,171
Securities Owned and Sold, Not yet Purchased, at Fair Value, Security Owned, Including Disposal Group Securities Owned	561,683	610,517
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	17,983	23,977
Securities Sold	10,286	5,283
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	28,393	25,190
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	17,152	$ 17,497
Securities Sold	$ 13,647