Pension and Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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	$ 233	$ 279	$ 700	$ 834
Interest cost	1,302	1,357	3,906	4,072
Contractual termination cost	0	32	0	32
Expected return on plan assets	(2,216)	(2,141)	(6,650)	(6,422)
Amortization of unrecognized net (gain) loss	578	585	1,734	1,729
Settlement	322	0	322	0
Net periodic benefit cost (income)	219	112	12	245
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	4	7
Interest cost	24	33	70	99
Amortization of prior service cost	0	29	0	88
Amortization of unrecognized net (gain) loss	(41)	(24)	(121)	(75)
Net periodic benefit cost (income)	$ (16)	$ 40	$ (47)	$ 119