Retirement Plans (Net Periodic Pension Cost (Income)) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
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	$ 1,247	$ 1,168	$ 1,133
Interest cost	4,603	4,795	4,779
Expected return on plan assets	(7,794)	(6,925)	(6,328)
Amortization of prior service credit	(1)	(1)	(2)
Amortization of net actuarial loss (gain)	3,670	2,382	1,073
Net cost (income)	1,725	1,419	655
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	18	0	0
Interest cost	143	146	167
Expected return on plan assets	0	0	0
Amortization of prior service credit	343	(300)	(325)
Amortization of net actuarial loss (gain)	(73)	(28)	(31)
Net cost (income)	$ 431	$ (182)	$ (189)