Noninterest Income and Noninterest Expense - Net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Pension Plan
Components of net periodic benefit cost
Interest cost	$ 1,880	$ 1,906
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Noninterest Expense	Other Noninterest Expense
Expected return on plan assets	$ (876)	$ (878)
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Noninterest Expense	Other Noninterest Expense
Recognized net actuarial loss (gain)	$ 383	$ 503
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Amortization of Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Noninterest Expense	Other Noninterest Expense
Total net periodic benefit cost	$ 1,387	$ 1,531
Other Postretirement Benefits Plan
Components of net periodic benefit cost
Interest cost	225	211
Recognized net actuarial loss (gain)	(301)	(322)
Total net periodic benefit cost	93	26
Other Postretirement Benefits Plan | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	$ 169	$ 137