Pension and Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
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	$ 234	$ 277	$ 467	$ 554
Interest cost	1,301	1,358	2,603	2,716
Expected return on plan assets	(2,217)	(2,140)	(4,434)	(4,281)
Amortization of unrecognized net (gain) loss	579	572	1,157	1,144
Net periodic benefit cost (income)	(103)	67	(207)	133
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	2	2	4
Interest cost	23	33	47	66
Amortization of prior service cost	0	29	0	58
Amortization of unrecognized net (gain) loss	(40)	(25)	(81)	(49)
Net periodic benefit cost (income)	$ (16)	$ 39	$ (32)	$ 79