Pension and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost (Income)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
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	$ 273	$ 250
Interest cost	1,332	1,312
Expected return on plan assets	(2,161)	(2,078)
Amortization of prior service cost	(1)	0
Amortization of unrecognized net loss (gain)	1,056	569
Net periodic benefit cost	499	53
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	4	5
Interest cost	33	35
Amortization of prior service cost	32	32
Amortization of unrecognized net loss (gain)	0	(26)
Net periodic benefit cost	$ 69	$ 46