Pension and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
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	$ 277	$ 273	$ 554	$ 545
Interest cost	1,358	1,332	2,716	2,664
Expected return on plan assets	(2,140)	(2,161)	(4,281)	(4,322)
Amortization of prior service cost	0	(1)	0	(2)
Amortization of unrecognized net loss (gain)	572	1,056	1,144	2,112
Net periodic benefit cost	67	499	133	997
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	2	4	4	8
Interest cost	33	33	66	67
Amortization of prior service cost	29	32	58	65
Amortization of unrecognized net loss (gain)	(25)	0	(49)	(1)
Net periodic benefit cost	$ 39	$ 69	$ 79	$ 139