Investment Securities - Contractual Maturities of Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less	$ 8,226
Due after one year through five years	508,645
Due after five years through ten years	1,067,818
Due after ten years	2,732,288
Total securities available-for-sale	4,316,977	$ 3,688,091
Fair Value
Due in one year or less	8,310
Due after one year through five years	532,309
Due after five years through ten years	1,135,249
Due after ten years	2,856,746
Total securities available-for-sale	$ 4,532,614	$ 3,797,187