Retirement Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 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	$ 2,345	$ 2,184
Expected return on plan assets	(2,190)	(2,220)	(4,381)	(4,439)
Amortization of unrecognized net (gain) loss	141	178	281	355
Net periodic benefit cost (income)	(877)	(950)	(1,755)	(1,900)
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	1	1
Interest cost	21	20	43	41
Amortization of unrecognized net (gain) loss	(45)	(36)	(90)	(71)
Net periodic benefit cost (income)	$ (24)	$ (15)	$ (46)	$ (29)