Pension and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 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	$ 818	$ 751
Interest cost	1,331	1,312	3,995	3,936
Expected return on plan assets	(2,161)	(2,078)	(6,483)	(6,235)
Amortization of prior service cost	(2)	0	(4)	(1)
Amortization of unrecognized net loss (gain)	1,057	569	3,169	1,707
Net periodic benefit cost	498	53	1,495	158
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	12	14
Interest cost	33	36	100	107
Amortization of prior service cost	32	32	97	97
Amortization of unrecognized net loss (gain)	0	(26)	(1)	(78)
Net periodic benefit cost	$ 69	$ 47	$ 208	$ 140