Accumulated Other Comprehensive Income/(Loss) (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified		$ (21,923)	$ (18,536)	$ (40,197)	$ (55,079)
Income tax effect		12,243	6,762	20,398	21,435
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		(9,658)	(11,812)	(19,699)	(33,652)
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	[1]	587	480	1,540	1,431
Income tax effect		(229)	(187)	(601)	(558)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		358	293	939	873
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		(1,110)	(1,108)	(3,330)	(3,325)
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		1,462	1,343	4,428	4,026
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	[2]	352	235	1,098	701
Income tax effect		(123)	(94)	(384)	(280)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ 229	$ 141	$ 714	$ 421

[1] Included in Interest expense.

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