Pension and Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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
Interest cost	1,092	1,302
Expected return on plan assets	(2,220)	(2,217)
Amortization of unrecognized net (gain) loss	178	578
Net periodic benefit cost (income)	(950)	(104)
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
Interest cost	20	24
Amortization of unrecognized net (gain) loss	(36)	(41)
Net periodic benefit cost (income)	$ (15)	$ (16)