Accumulated Other Comprehensive Income (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	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, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Payments made on interest rate swaps included in earnings														$ 15,541	$ 11,841	$ 12,254
Total pretax amount reclassified														(78,266)	(51,478)	(67,500)
Income tax effect														25,454	(5,787)	25,751
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ (15,800)	$ (13,100)	$ (10,400)	$ (13,500)	$ (37,600)	$ (9,700)	$ 2,200	$ (12,200)	$ (8,000)	$ (11,800)	$ (11,200)	$ (10,600)	(52,733)	(57,279)	(41,569)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivative Valuation Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Payments made on interest rate swaps included in earnings	[1]													2,400	1,988	1,914
Income tax effect														(912)	(755)	(746)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income														1,488	1,233	1,168
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Settlements and Curtailments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified														51	103	8,377
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified														(4,450)	(4,440)	(4,436)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified														5,102	5,932	5,329
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension and Postretirement Liability Adjustments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total pretax amount reclassified	[2]													703	1,595	9,270
Income tax effect														21	(270)	(3,540)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income														$ 724	$ 1,325	$ 5,730

[1] Included in Interest expense are payments related to the interest rate swap agreements and amortization of swap buyouts (see Note 15).

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