Retirement Plans (Net Periodic Pension Cost (Income)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	$ 0	$ 639	$ 1,041
Interest cost	4,356	4,966	5,335
Expected return on plan assets	(8,879)	(8,938)	(8,562)
Amortization of prior service credit	0	0	0
Amortization of net actuarial loss (gain)	710	1,837	2,259
Curtailment	0	1	0
Settlement	0	0	0
Net cost (income)	(3,813)	(1,495)	73
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	3	5	9
Interest cost	82	94	132
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	117
Amortization of net actuarial loss (gain)	(143)	(162)	(100)
Curtailment	0	0	0
Settlement	0	0	0
Net cost (income)	(58)	(63)	158
SERP
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	0	61	77
Interest cost	12	69	92
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	0
Amortization of net actuarial loss (gain)	(7)	77	78
Curtailment	0	0	0
Settlement	0	322	120
Net cost (income)	$ 5	$ 529	$ 367