Accumulated Other Comprehensive Income/(Loss) (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified		$ 2,484	$ (18,393)	$ (18,274)	$ (36,543)
Income tax effect		(364)	7,216	8,155	14,673
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		2,172	(11,219)	(10,041)	(21,840)
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]	467	473	953	951
Income tax effect		(182)	(184)	(372)	(371)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		285	289	581	580
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		(1,111)	(1,108)	(2,220)	(2,217)
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		1,461	1,355	2,966	2,683
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]	350	247	746	466
Income tax effect		(122)	(98)	(261)	(186)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ 228	$ 149	$ 485	$ 280

[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).