Retirement Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Pension Plans
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Interest cost	$ 1,172	$ 1,092
Expected return on plan assets	(2,190)	(2,220)
Amortization of unrecognized net (gain) loss	141	178
Net periodic benefit cost (income)	(877)	(950)
Postretirement Benefit Plan
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	0	1
Interest cost	21	20
Amortization of unrecognized net (gain) loss	(45)	(36)
Net periodic benefit cost (income)	$ (24)	$ (15)