Summary of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (1,866,245)	$ (1,876,160)	$ (1,760,434)	$ (1,750,699)	$ (1,876,160)	$ (1,750,699)
Net other comprehensive loss (income)	(49)	(76)	(78)	(69)	(125)	(147)
Ending Balance	(1,855,732)	(1,866,245)	(1,794,850)	(1,760,434)	(1,855,732)	(1,794,850)
Gain on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,393)	(3,614)	(4,282)	(4,501)	(3,614)	(4,501)
Amounts reclassified from accumulated other comprehensive loss	224	221	221	219
Net other comprehensive loss (income)	224	221	221	219
Ending Balance	(3,169)	(3,393)	(4,061)	(4,282)	(3,169)	(4,061)
Defined Benefit Pension Plan [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	14,600	14,897	14,762	15,050	14,897	15,050
Amounts reclassified from accumulated other comprehensive loss	(273)	(297)	(299)	(288)
Net other comprehensive loss (income)	(273)	(297)	(299)	(288)
Ending Balance	14,327	14,600	14,463	14,762	14,327	14,463
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	11,207	11,283	10,480	10,549	11,283	10,549
Amounts reclassified from accumulated other comprehensive loss	(49)	(76)	(78)	(69)
Net other comprehensive loss (income)	(49)	(76)	(78)	(69)
Ending Balance	$ 11,158	$ 11,207	$ 10,402	$ 10,480	$ 11,158	$ 10,402