Investment Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due after five years through ten years	$ 141,285
Due after ten years	1,500,200
Total	1,641,485	$ 955,542
Fair Value
Due after five years through ten years	137,970
Due after ten years	1,469,214
Fair Value	1,607,184	957,183
Amortized Cost
Due in one year or less	10,948
Due after one year through five years	102,771
Due after five years through ten years	752,981
Due after ten years	3,765,339
Amortized Cost	4,632,039	3,268,037
Fair Value
Due in one year or less	10,998
Due after one year through five years	105,345
Due after five years through ten years	757,174
Due after ten years	3,758,037
Fair Value	$ 4,631,554	$ 3,330,672