Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Amortized Cost	$ 1,599,894	$ 1,293,239
Fair Value
Total available-for-sale debt securities	1,627,193	1,298,587
Debt securities
Amortized Cost
Due in one year or less	54,484	107,975
Due after one year through five years	379,044	270,477
Due after five years through ten years	228,572	77,710
Due after ten years	66,694	12,397
Amortized Cost	728,794	468,559
Fair Value
Due in one year or less	55,008	108,089
Due after one year through five years	388,132	274,798
Due after five years through ten years	229,107	79,165
Due after ten years	67,358	12,494
Total available-for-sale debt securities	739,605	474,546
Mortgage-backed securities
Amortized Cost
Amortized Cost	871,100	824,680
Fair Value
Total available-for-sale debt securities	$ 887,588	$ 824,041