Summary of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (1,795,421)	$ (1,800,502)	$ (1,866,245)	$ (1,876,160)	$ (1,800,502)	$ (1,876,160)
Net other comprehensive loss (income)	(61)	(42)	(49)	(76)	(103)	(125)
Ending Balance	(1,780,463)	(1,795,421)	(1,855,732)	(1,866,245)	(1,780,463)	(1,855,732)
Gain on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2,505)	(2,724)	(3,393)	(3,614)	(2,724)	(3,614)
Amounts reclassified from accumulated other comprehensive loss	221	219	224	221
Net other comprehensive loss (income)	221	219	224	221
Ending Balance	(2,284)	(2,505)	(3,169)	(3,393)	(2,284)	(3,169)
Defined Benefit Pension Plan [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	14,513	14,774	14,600	14,897	14,774	14,897
Amounts reclassified from accumulated other comprehensive loss	(282)	(261)	(273)	(297)
Net other comprehensive loss (income)	(282)	(261)	(273)	(297)
Ending Balance	14,231	14,513	14,327	14,600	14,231	14,327
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	12,008	12,050	11,207	11,283	12,050	11,283
Amounts reclassified from accumulated other comprehensive loss	(61)	(42)	(49)	(76)
Net other comprehensive loss (income)	(61)	(42)	(49)	(76)
Ending Balance	$ 11,947	$ 12,008	$ 11,158	$ 11,207	$ 11,947	$ 11,158