Borrowings - Senior notes and Subordinate Notes (Details) - USD ($) $ in Thousands	Jun. 01, 2020	May 25, 2017	Mar. 31, 2021	Dec. 31, 2020
Long-term debt
Unamortized debt issuance cost			$ 2,787	$ 2,965
3.75% notes maturing May 25, 2022 | Senior notes
Long-term debt
Issuance of debt		$ 40,000
Interest rate (as a percent)		3.75%
Unamortized debt issuance cost			157	191
4.75% notes maturing May 25, 2027 | Subordinated debt
Long-term debt
Issuance of debt		$ 60,000
Fixed payment term of note		5 years
Duration of fixed interest rate		5 years
Unamortized debt issuance cost			625	651
4.75% notes maturing May 25, 2027 | First Five Years | Subordinated debt
Long-term debt
Interest rate (as a percent)		4.75%
4.75% notes maturing May 25, 2027 | Three-month LIBOR | Subordinated debt
Long-term debt
Floating interest rate margin (as a percent)		2.919%
5.25% notes maturing June 1, 2030 | Subordinated debt
Long-term debt
Issuance of debt	$ 125,000
Duration of fixed interest rate	5 years
Floating interest rate margin (as a percent)	5.11%
Unamortized debt issuance cost			$ 2,005	$ 2,123
5.25% notes maturing June 1, 2030 | First Five Years | Subordinated debt
Long-term debt
Interest rate (as a percent)	5.25%