Summary of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (1,855,732)	$ (1,866,245)	$ (1,876,160)	$ (1,794,850)	$ (1,760,434)	$ (1,750,699)	$ (1,876,160)	$ (1,750,699)
Cumulative effect adjustment of accounting standards	1,855,732	1,855,732	1,866,245	1,852,477	1,794,850	1,760,434	1,876,160	1,852,477
Net other comprehensive loss (income)	(4)	(49)	(76)	(82)	(78)	(69)	(129)	(229)
Ending Balance	(1,837,829)	(1,855,732)	(1,866,245)	(1,852,477)	(1,794,850)	(1,760,434)	(1,837,829)	(1,852,477)
Gain on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(3,169)	(3,393)	(3,614)	(4,061)	(4,282)	(4,501)	(3,614)	(4,501)
Cumulative effect adjustment of accounting standards	2,948	3,169	3,393	3,838	4,061	4,282	2,948	3,838
Amounts reclassified from accumulated other comprehensive loss	221	224	221	223	221	219
Net other comprehensive loss (income)	221	224	221	223	221	219
Ending Balance	(2,948)	(3,169)	(3,393)	(3,838)	(4,061)	(4,282)	(2,948)	(3,838)
Defined Benefit Pension Plan [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	14,327	14,600	14,897	14,463	14,762	15,050	14,897	15,050
Cumulative effect adjustment of accounting standards	(14,102)	(14,327)	(14,600)	(14,158)	(14,463)	(14,762)	(14,102)	(14,158)
Amounts reclassified from accumulated other comprehensive loss	(225)	(273)	(297)	(305)	(299)	(288)
Net other comprehensive loss (income)	(225)	(273)	(297)	(305)	(299)	(288)
Ending Balance	14,102	14,327	14,600	14,158	14,463	14,762	14,102	14,158
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	11,158	11,207	11,283	10,402	10,480	10,549	11,283	10,549
Cumulative effect adjustment of accounting standards	(11,154)	(11,158)	(11,207)	(10,320)	(10,402)	(10,480)	(11,154)	(10,320)
Amounts reclassified from accumulated other comprehensive loss	(4)	(49)	(76)	(82)	(78)	(69)
Net other comprehensive loss (income)	(4)	(49)	(76)	(82)	(78)	(69)
Ending Balance	$ 11,154	11,158	11,207	$ 10,320	$ 10,402	$ 10,480	$ 11,154	$ 10,320
Cumulative Effect, Period of Adoption, Adjustment [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance		711
Cumulative effect adjustment of accounting standards		$ (711)	(711)
Ending Balance			$ 711