Investment Securities - Debt Maturities (Details) - Investment securities: - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Amortized Cost
Due within one year	$ 1,085,097
Due after one year through five years	693,669
Due after five years through ten years	1,205,894
Due after ten years	815,877
Amortized cost of available-for-sale investment securities	3,800,537	$ 3,843,666
Fair Value
Due within one year	1,085,613
Due after one year through five years	694,736
Due after five years through ten years	1,212,911
Due after ten years	816,372
Fair Value	$ 3,809,632	$ 3,853,692
Weighted- Average Yield
Due within one year	1.90%
Due after one year through five years	0.77%
Due after five years through ten years	0.56%
Due after ten years	0.63%
Total, Weighted-Average Yield	1.00%