Noninterest Income and Noninterest Expense - Net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Pension Plan
Components of net periodic benefit cost
Interest cost	$ 1,879	$ 1,905	$ 3,759	$ 3,811
Expected return on plan assets	(876)	(878)	(1,752)	(1,756)
Recognized net actuarial loss (gain)	383	503	766	1,006
Total net periodic benefit cost	1,386	1,530	2,773	3,061
Other Postretirement Benefits Plan
Components of net periodic benefit cost
Interest cost	225	211	450	422
Recognized net actuarial loss (gain)	(301)	(322)	(602)	(644)
Total net periodic benefit cost	93	26	186	52
Other Postretirement Benefits Plan | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	$ 169	$ 137	$ 338	$ 274