Accumulated Other Comprehensive Income (AOCI) (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Expense related to interest rate swaps included in Income		$ 4,625	$ 4,238	$ 9,060	$ 6,602
Total pretax amount reclassified		3,012	16,183	20,536	36,542
Income tax effect		1,779	6,082	8,329	13,125
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		(1,117)	(10,367)	(11,956)	(23,868)
Prior service credit		(1,113)	(1,112)	(2,226)	(2,225)
Net actuarial loss		1,353	1,296	2,700	2,574
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	[1]	240	184	474	349
Income tax effect		(72)	(56)	(142)	(105)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		168	128	332	244
Reclassification out of Accumulated Other Comprehensive Income [Member] | Derivative valuation adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Expense related to interest rate swaps included in Income	[2]	343	305	943	586
Income tax effect		(130)	(116)	(358)	(223)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ 213	$ 189	$ 585	$ 363

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

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