Investment Securities - Amortized cost and fair value by contractual maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Amortized Cost
Due in one year or less	$ 50,000
Due after one year through five years	50,994
Due after five years through ten years	407,416
Due after ten years	2,025,303
Total	2,533,713	$ 2,683,241
Fair Value
Due in one year or less	48,682
Due after one year through five years	44,922
Due after five years through ten years	341,124
Due after ten years	1,577,672
Fair Value	2,012,400	2,250,168
Amortized Cost
Due in one year or less	234,334
Due after one year through five years	256,202
Due after five years through ten years	1,342,414
Due after ten years	3,876,320
Amortized Cost	5,709,270	6,216,141
Fair Value
Due in one year or less	231,348
Due after one year through five years	239,370
Due after five years through ten years	1,104,205
Due after ten years	3,048,695
Fair Value	$ 4,623,618	$ 5,326,822