Accumulated Other Comprehensive Income (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Payments made on interest rate swaps included in earnings														$ 11,841	$ 12,254	$ 15,227
Total pretax amount reclassified														(51,478)	(67,500)	(31,076)
Income tax effect														(5,787)	25,751	13,372
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ (37,600)	$ (9,700)	$ 2,200	$ (12,200)	$ (8,000)	$ (11,800)	$ (11,200)	$ (10,600)	$ (8,700)	$ (4,700)	$ 7,400	$ (11,500)	(57,279)	(41,569)	(17,517)
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]													1,988	1,914	1,900
Income tax effect														(755)	(746)	(741)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income														1,233	1,168	$ 1,159
Pension/Postretirement Settlements and Curtailments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified														103	8,377
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														$ (4,440)	(4,436)	$ (3,905)
Accumulated Defined Benefit Plans Adjustment, Net Transition Asset (Obligation) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified																70
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														$ 5,932	5,329	6,512
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]													1,595	9,270	2,677
Income tax effect														(270)	(3,540)	(451)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income														$ 1,325	$ 5,730	$ 2,226

[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).