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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Pension Plans
Components of net periodic benefit cost (income) for the Corporation's qualified and nonqualified pension plans and nonqualified postretirement benefits plan
Service cost	$ 279	$ 273	$ 834	$ 818
Interest cost	1,357	1,331	4,072	3,995
Contractual termination cost	32	0	32	0
Expected return on plan assets	(2,141)	(2,161)	(6,422)	(6,483)
Amortization of prior service cost	0	(2)	0	(4)
Amortization of unrecognized net loss (gain)	585	1,057	1,729	3,169
Net periodic benefit cost	112	498	245	1,495
Postretirement Benefit Plan
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	7	12
Interest cost	33	33	99	100
Amortization of prior service cost	29	32	88	97
Amortization of unrecognized net loss (gain)	(24)	0	(75)	(1)
Net periodic benefit cost	$ 40	$ 69	$ 119	$ 208