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, 2013	Mar. 31, 2012
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	$ 325	$ 303
Interest cost	1,167	1,215
Expected return on plan assets	(1,948)	(1,731)
Amortization of unrecognized net loss (gain)	946	614
Net periodic benefit cost (income)	490	401
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
Interest cost	75	37
Amortization of prior service credit	0	(75)
Amortization of unrecognized net loss (gain)	0	(7)
Net periodic benefit cost (income)	$ 75	$ (45)