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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
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	$ 250	$ 326	$ 751	$ 976
Interest cost	1,312	1,166	3,936	3,500
Expected return on plan assets	(2,078)	(1,949)	(6,235)	(5,845)
Amortization of prior service cost	0	0	(1)	(1)
Amortization of unrecognized net loss (gain)	569	946	1,707	2,838
Net periodic benefit cost	53	489	158	1,468
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	5	14	14
Interest cost	36	36	107	108
Amortization of prior service cost	32	27	97	233
Amortization of unrecognized net loss (gain)	(26)	(18)	(78)	(55)
Net periodic benefit cost	$ 47	$ 50	$ 140	$ 300