Investment Securities - Contractual Maturities of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less	$ 40,903
Due after one year through five years	578,943
Due after five years through ten years	2,991,339
Due after ten years	5,529,139
Total securities available-for-sale	9,140,324	$ 4,996,970
Fair Value
Due in one year or less	41,218
Due after one year through five years	606,629
Due after five years through ten years	3,029,128
Due after ten years	5,599,951
Total securities available-for-sale	$ 9,276,926	$ 5,235,591