Investment Securities - Contractual Maturities of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 27,259
Due after one year through five years	579,444
Due after five years through ten years	2,657,145
Due after ten years	3,733,687
Total securities available-for-sale	6,997,535	$ 4,996,970
Fair Value
Due in one year or less	27,267
Due after one year through five years	608,063
Due after five years through ten years	2,713,777
Due after ten years	3,849,501
Total securities available-for-sale	$ 7,198,608	$ 5,235,591