Accumulated Other Comprehensive Income/(Loss) (Schedule of Items Reclassified to Statement of Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified	$ (20,758)		$ (18,150)
Income tax effect	8,519		7,457
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income	(12,213)		(10,621)
Derivative Valuation Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Payments made on interest rate swaps included in earnings	486	[1]	478	[1]
Income tax effect	(190)		(186)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income	296		292
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified	(1,109)		(1,109)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified	1,505		1,328
Pension and Postretirement Liability Adjustments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified	396	[2]	219	[2]
Income tax effect	(139)		(88)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income	$ 257		$ 131

[1] Included in Interest expense.

[2] These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see Note 4).