Pension and Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
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
Service cost	$ 0	$ 233	$ 0	$ 700
Interest cost	1,092	1,302	3,276	3,906
Expected return on plan assets	(2,220)	(2,216)	(6,659)	(6,650)
Amortization of unrecognized net (gain) loss	178	578	533	1,734
Settlement	0	322	0	322
Net periodic benefit cost (income)	(950)	219	(2,850)	12
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	1	1	2	4
Interest cost	20	24	61	70
Amortization of unrecognized net (gain) loss	(36)	(41)	(107)	(121)
Net periodic benefit cost (income)	$ (15)	$ (16)	$ (44)	$ (47)