Retirement Plans (Net Periodic Pension Cost (Income)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	$ 639	$ 1,041	$ 1,021
Interest cost	4,966	5,335	5,242
Expected return on plan assets	(8,938)	(8,562)	(8,645)
Amortization of prior service credit	0	0	(5)
Amortization of net actuarial loss (gain)	1,837	2,259	3,962
Curtailment	1	0	0
Settlement	0	0	0
Net cost (income)	(1,495)	73	1,575
Postretirement Plan
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	5	9	16
Interest cost	94	132	133
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	117	130
Amortization of net actuarial loss (gain)	(162)	(100)	(1)
Curtailment	0	0	0
Settlement	0	0	0
Net cost (income)	(63)	158	278
SERP
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	61	77	70
Interest cost	69	92	85
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	0
Amortization of net actuarial loss (gain)	77	78	262
Curtailment	0	0	0
Settlement	322	120	0
Net cost (income)	$ 529	$ 367	$ 417