Debt Securities - Credit Quality Indictors for HTM Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
HTM debt securities
Debt securities, held-to-maturity	$ 588,751	$ 189,836
State and local governments
HTM debt securities
Debt securities, held-to-maturity	588,751	189,836
State and local governments | Moody's: Aaa | S&P: AAA | Investment Grade
HTM debt securities
Debt securities, held-to-maturity	91,831	39,022
State and local governments | Moody's: Aa1, Aa2, Aa3 | S&P: AA+, AA, AA- | Investment Grade
HTM debt securities
Debt securities, held-to-maturity	471,159	123,664
State and local governments | Moody's: A1, A2, A3 | S&P: A+, A, A- | Investment Grade
HTM debt securities
Debt securities, held-to-maturity	$ 25,761	$ 27,150