Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
RAP
Net period benefit cost for the pension and postretirement plans
Defined Benefit Plan, Description	The Corporation has a noncontributory defined benefit RAP, covering substantially all employees who meet the eligibility requirements. The benefits are based primarily on years of service and the employee’s eligible compensation paid. Employees of acquired entities generally participate in the RAP after consummation of the business combinations. Retirement plans of acquired entities are typically merged into the RAP depending on the terms of the merger agreement, and, as applicable, credit is usually applied to employees for years of service at the acquired institution for vesting and eligibility purposes.
Service cost	$ 874	$ 796
Interest cost	2,719	2,686
Expected return on plan assets	(8,650)	(8,202)
Amortization of prior service cost	(53)	(63)
Amortization of actuarial loss	0	37
Total net periodic pension cost	(5,111)	(4,746)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	18	20
Amortization of prior service cost	(19)	(19)
Amortization of actuarial loss	(7)	(7)
Total net periodic pension cost	$ (8)	$ (7)