Pension and Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	$ 234	$ 0	$ 467
Interest cost	1,092	1,301	2,184	2,603
Expected return on plan assets	(2,220)	(2,217)	(4,439)	(4,434)
Amortization of unrecognized net (gain) loss	178	579	355	1,157
Net periodic benefit cost (income)	(950)	(103)	(1,900)	(207)
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	1	2
Interest cost	20	23	41	47
Amortization of unrecognized net (gain) loss	(36)	(40)	(71)	(81)
Net periodic benefit cost (income)	$ (15)	$ (16)	$ (29)	$ (32)