Investment Securities - Contractual Maturities of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 16,309
Due after one year through five years	571,182
Due after five years through ten years	1,963,578
Due after ten years	3,243,624
Total securities available-for-sale	5,794,693	$ 4,996,970
Fair Value
Due in one year or less	16,075
Due after one year through five years	598,902
Due after five years through ten years	1,989,510
Due after ten years	3,337,203
Total securities available-for-sale	$ 5,941,690	$ 5,235,591