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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
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	$ 325	$ 303	$ 650	$ 606
Interest cost	1,167	1,215	2,334	2,430
Expected return on plan assets	(1,948)	(1,732)	(3,896)	(3,463)
Amortization of prior service credit	(1)	(1)	(1)	(1)
Amortization of unrecognized net loss (gain)	946	615	1,892	1,229
Net periodic benefit cost (income)	489	400	979	801
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	0	9	0
Interest cost	36	36	72	73
Amortization of prior service credit	153	(75)	206	(150)
Amortization of unrecognized net loss (gain)	(18)	(7)	(37)	(14)
Net periodic benefit cost (income)	$ 175	$ (46)	$ 250	$ (91)