Pension and Other Postretirement Benefit Plans (Components of Net Periodic Benefit Cost (Income)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 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	$ 251	$ 325	$ 501	$ 650
Interest cost	1,312	1,167	2,624	2,334
Expected return on plan assets	(2,079)	(1,948)	(4,157)	(3,896)
Amortization of prior service cost	(1)	(1)	(1)	(1)
Amortization of unrecognized net loss (gain)	569	946	1,138	1,892
Net periodic benefit cost	52	489	105	979
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	9	9
Interest cost	36	36	71	72
Amortization of prior service cost	33	153	65	206
Amortization of unrecognized net loss (gain)	(26)	(18)	(52)	(37)
Net periodic benefit cost	$ 47	$ 175	$ 93	$ 250