Fair Value Measurements - Securities Owned and Securities Sold, But Not Yet Purchased at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
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	$ 689,046	$ 634,504
Securities Sold	97,371	71,958
Securities owned and sold, fair value	689,046	634,504
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	14,543	8,111
Securities Sold	5,805	2,515
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	20,136	18,520
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	23,074	13,778
Securities Sold	$ 6,741	$ 8,462