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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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	$ 251	$ 545	$ 501
Interest cost	1,332	1,312	2,664	2,624
Expected return on plan assets	(2,161)	(2,079)	(4,322)	(4,157)
Amortization of prior service cost	(1)	(1)	(2)	(1)
Amortization of unrecognized net loss (gain)	1,056	569	2,112	1,138
Net periodic benefit cost	499	52	997	105
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	4	8	9
Interest cost	33	36	67	71
Amortization of prior service cost	32	33	65	65
Amortization of unrecognized net loss (gain)	0	(26)	(1)	(52)
Net periodic benefit cost	$ 69	$ 47	$ 139	$ 93