Retirement Plans (Net Periodic Pension Cost (Income)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 1,041	$ 1,021	$ 944
Interest cost	5,335	5,242	5,167
Expected return on plan assets	(8,562)	(8,645)	(8,314)
Amortization of prior service credit	0	(5)	(1)
Amortization of net actuarial loss (gain)	2,259	3,962	2,159
Net cost (income)	73	1,575	(45)
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	9	16	18
Interest cost	132	133	142
Expected return on plan assets	0	0	0
Amortization of prior service credit	117	130	130
Amortization of net actuarial loss (gain)	(100)	(1)	(104)
Net cost (income)	$ 158	$ 278	$ 186