Accumulated Other Comprehensive Income (AOCI) (Schedule of Items Reclassified to Statement of Income) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Expense related to interest rate swaps included in Income		$ 4,784	$ 4,356	$ 13,843	$ 10,957
Total pretax amount reclassified		19,029	20,897	39,565	57,439
Income tax effect		3,809	7,488	12,138	20,613
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		(15,269)	(13,069)	(27,225)	(36,937)
Prior service credit		(1,113)	(1,113)	(3,339)	(3,338)
Net actuarial loss		1,350	1,296	4,050	3,870
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]	295	1,100	1,238	1,686
Income tax effect		(112)	(418)	(470)	(641)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		183	682	768	1,045
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]	237	183	711	532
Income tax effect		(71)	(55)	(213)	(160)
Effect on net income due to items reclassified from Accumulated Other Comprehensive Income		$ 166	$ 128	$ 498	$ 372

[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).