Noninterest Income and Noninterest Expense - Net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Pension Plan [Member]
Components of net periodic benefit cost
Interest cost	$ 1,905	$ 2,059	$ 3,811	$ 4,118
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	$ (878)	$ (882)	(1,756)	(1,765)
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)	$ 503	$ 719	1,006	1,438
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	$ 1,530	$ 1,896	3,061	3,791
Other Postretirement Benefits Plan [Member]
Components of net periodic benefit cost
Interest cost	$ 211	$ 220	422	440
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Noninterest Expense	Other Noninterest Expense
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)	$ (322)	$ (379)	(644)	(758)
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	$ 26	$ (12)	52	(24)
Other Postretirement Benefits Plan [Member] | Salaries and employee benefits
Components of net periodic benefit cost
Service cost	$ 137	$ 147	$ 274	$ 294