Pension and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Pension Plans [Member]
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	$ 277	$ 273
Interest cost	1,358	1,332
Expected return on plan assets	(2,141)	(2,161)
Amortization of prior service cost	0	(1)
Amortization of unrecognized net loss (gain)	572	1,056
Net periodic benefit cost	66	499
Postretirement Benefit 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	2	4
Interest cost	33	33
Amortization of prior service cost	29	32
Amortization of unrecognized net loss (gain)	(24)	0
Net periodic benefit cost	$ 40	$ 69