Retirement Plans (Net Periodic Pension Cost (Income)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan [Member]
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	$ 1,021	$ 944	$ 1,247
Interest cost	5,242	5,167	4,603
Expected return on plan assets	(8,645)	(8,314)	(7,794)
Amortization of prior service credit	(5)	(1)	(1)
Amortization of net actuarial loss (gain)	3,962	2,159	3,670
Net cost (income)	1,575	(45)	1,725
Postretirement Plan [Member]
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	16	18	18
Interest cost	133	142	143
Expected return on plan assets	0	0	0
Amortization of prior service credit	130	130	343
Amortization of net actuarial loss (gain)	(1)	(104)	(73)
Net cost (income)	$ 278	$ 186	$ 431