Retirement Plans (Net Periodic Pension Cost (Income)) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension 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	$ 1,168	$ 1,133	$ 1,228
Interest cost	4,795	4,779	4,785
Expected return on plan assets	(6,925)	(6,328)	(5,724)
Amortization of prior service credit	(1)	(2)	(2)
Amortization of net actuarial loss (gain)	2,382	1,073	491
Net cost (income)	1,419	655	778
Postretirement 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	0	0	0
Interest cost	146	167	217
Expected return on plan assets	0	0	0
Amortization of prior service credit	(300)	(325)	(324)
Amortization of net actuarial loss (gain)	(28)	(31)	0
Net cost (income)	$ (182)	$ (189)	$ (107)