Summary of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (1,876,160)	$ (1,750,699)	$ (1,809,842)
Cumulative effect adjustment of new accounting standards	1,876,160	1,750,699	1,750,699
Net other comprehensive loss (income)			2,838
Net other comprehensive loss (income)	767	734
Ending Balance	(1,800,502)	(1,876,160)	(1,750,699)
Gain on Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(1,541)
Cumulative effect adjustment of new accounting standards			1,541
Gain on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,614)	(4,501)	(7,424)
Cumulative effect adjustment of new accounting standards	2,724	3,614	4,501
Change excluding amounts reclassified from accumulated other comprehensive loss			(1,379)
Amounts reclassified from accumulated other comprehensive loss	890	887	4,302
Net other comprehensive loss (income)	890	887	2,923
Ending Balance	(2,724)	(3,614)	(4,501)
Defined Benefit Pension Plan [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	14,897	15,050	15,135
Cumulative effect adjustment of new accounting standards	(14,774)	(14,897)	(15,050)
Change excluding amounts reclassified from accumulated other comprehensive loss	898	1,044	1,143
Amounts reclassified from accumulated other comprehensive loss	(1,021)	(1,197)	(1,228)
Net other comprehensive loss (income)	(123)	(153)	(85)
Ending Balance	14,774	14,897	15,050
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	11,283	10,549	6,170
Cumulative effect adjustment of new accounting standards	(12,050)	(11,283)	(10,549)
Change excluding amounts reclassified from accumulated other comprehensive loss	898	1,044	(236)
Amounts reclassified from accumulated other comprehensive loss	(131)	(310)	3,074
Net other comprehensive loss (income)			2,838
Net other comprehensive loss (income)	767	734
Ending Balance	12,050	11,283	10,549
Cumulative Effect, Period of Adoption, Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		(3,956)
Cumulative effect adjustment of new accounting standards	(711)	3,956	3,956
Ending Balance	$ 711		(3,956)
Cumulative Effect, Period of Adoption, Adjustment [Member] | Gain on Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		1,541
Cumulative effect adjustment of new accounting standards		(1,541)	(1,541)
Ending Balance			1,541
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		1,541
Cumulative effect adjustment of new accounting standards		$ (1,541)	(1,541)
Ending Balance			$ 1,541