Retirement Plans, Components of Net Periodic Benefit Cost for the Pension and Postretirement Tables (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
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 participation requirements. The benefits are based primarily on years of service and the employee’s compensation paid. Employees of acquired entities generally participate in the RAP after consummation of the business combinations. Any retirement plans of acquired entities are typically merged into the RAP after completion of the mergers, and credit is usually given to employees for years of service at the acquired institution for vesting and eligibility purposes.
Service cost	$ 800	$ 906	$ 2,392	$ 2,752
Interest cost	2,794	1,820	8,165	5,364
Expected return on plan assets	(8,243)	(6,706)	(24,647)	(20,177)
Amortization of prior service cost	(63)	(63)	(188)	(188)
Amortization of actuarial loss	0	347	37	494
Total net periodic pension cost	(4,713)	(3,696)	(14,241)	(11,754)
Postretirement Plan
Net period benefit cost for the pension and postretirement plans
Interest cost	20	13	59	40
Amortization of prior service cost	(19)	(19)	(56)	(56)
Amortization of actuarial loss	(7)	0	(22)	0
Total net periodic pension cost	$ (6)	$ (6)	$ (19)	$ (17)