Accumulated Other Comprehensive Income (AOCI) (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Expense related to interest rate swaps included in Income		$ 4,670	$ 4,435
Total pretax amount reclassified		15,091	17,524
Income tax effect		4,609	6,550
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		(10,245)	(10,839)
Prior service credit		(1,114)	(1,113)
Net actuarial loss		1,297	1,347
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	[1]	180	600
Income tax effect		(43)	(228)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		137	372
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]	183	234
Income tax effect		(55)	(70)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ 128	$ 164

[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).