Pension and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost (Income)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 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	$ 326	$ 304	$ 976	$ 910
Interest cost	1,166	1,214	3,500	3,644
Expected return on plan assets	(1,949)	(1,731)	(5,845)	(5,194)
Amortization of prior service cost (credit)	0	0	(1)	(1)
Amortization of unrecognized net loss (gain)	946	614	2,838	1,843
Net periodic benefit cost (income)	489	401	1,468	1,202
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	5	0	14	0
Interest cost	36	37	108	110
Amortization of prior service cost (credit)	27	(75)	233	(225)
Amortization of unrecognized net loss (gain)	(18)	(7)	(55)	(21)
Net periodic benefit cost (income)	$ 50	$ (45)	$ 300	$ (136)