Summary of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ (1,794,850)	$ (1,760,434)	$ (1,750,699)	$ (1,923,994)	$ (1,906,343)	$ (1,809,842)	$ (1,750,699)	$ (1,809,842)
Cumulative effect adjustment of accounting standards	0			0			0	1,541
Net other comprehensive loss (income)	(82)	(78)	(69)	(101)	(86)	1,983	(229)	1,796
Ending Balance	(1,852,477)	(1,794,850)	(1,760,434)	(1,922,721)	(1,923,994)	(1,906,343)	(1,852,477)	(1,922,721)
Gain on Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0	0	0	0	0	(1,541)	0	(1,541)
Cumulative effect adjustment of accounting standards						1,541
Change excluding amounts reclassified from accumulated other comprehensive loss
Amounts reclassified from accumulated other comprehensive loss
Net other comprehensive loss (income)	0	0	0	0	0	0
Ending Balance	0	0	0	0	0	0	0	0
Gain on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(4,061)	(4,282)	(4,501)	(4,949)	(5,170)	(7,424)	(4,501)	(7,424)
Change excluding amounts reclassified from accumulated other comprehensive loss						(1,379)
Amounts reclassified from accumulated other comprehensive loss	223	221	219	224	221	3,633
Net other comprehensive loss (income)	223	221	219	224	221	2,254
Ending Balance	(3,838)	(4,061)	(4,282)	(4,725)	(4,949)	(5,170)	(3,838)	(4,725)
Defined Benefit Pension Plan [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	14,463	14,762	15,050	14,557	14,864	15,135	15,050	15,135
Amounts reclassified from accumulated other comprehensive loss	(305)	(299)	(288)	(325)	(307)	(271)
Net other comprehensive loss (income)	(305)	(299)	(288)	(325)	(307)	(271)
Ending Balance	14,158	14,463	14,762	14,232	14,557	14,864	14,158	14,232
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	10,402	10,480	10,549	9,608	9,694	6,170	10,549	6,170
Cumulative effect adjustment of accounting standards						1,541
Change excluding amounts reclassified from accumulated other comprehensive loss						(1,379)
Amounts reclassified from accumulated other comprehensive loss	(82)	(78)	(69)	(101)	(86)	3,362
Net other comprehensive loss (income)	(82)	(78)	(69)	(101)	(86)	1,983
Ending Balance	$ 10,320	$ 10,402	$ 10,480	$ 9,507	$ 9,608	$ 9,694	$ 10,320	$ 9,507