Summary of Significant Accounting Policies (Schedule Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ (1,809,842)	$ (1,716,896)
Cumulative effect adjustment of accounting standards (see Note 2)	$ 0	$ 0	1,541	0
Net other comprehensive loss (income)	(86)	(57)	1,897	(1,260)
Ending Balance	(1,923,994)	(1,760,105)	(1,923,994)	(1,760,105)
Gain on Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(1,541)	(964)
Cumulative effect adjustment of accounting standards (see Note 2)			1,541
Change excluding amounts reclassified from accumulated other comprehensive loss				(456)
Amounts reclassified from accumulated other comprehensive loss
Net other comprehensive loss (income)			0	(456)
Ending Balance	0	(1,420)	0	(1,420)
Gain on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(7,424)	(6,403)
Change excluding amounts reclassified from accumulated other comprehensive loss			(1,379)	106
Amounts reclassified from accumulated other comprehensive loss			3,854	(164)
Net other comprehensive loss (income)			2,475	(58)
Ending Balance	(4,949)	(6,461)	(4,949)	(6,461)
Defined Benefit Pension Plan [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			15,135	16,528
Amounts reclassified from accumulated other comprehensive loss			(578)	(746)
Net other comprehensive loss (income)			(578)	(746)
Ending Balance	14,557	15,782	14,557	15,782
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			6,170	9,161
Cumulative effect adjustment of accounting standards (see Note 2)			1,541
Change excluding amounts reclassified from accumulated other comprehensive loss			(1,379)	(350)
Amounts reclassified from accumulated other comprehensive loss			3,276	(910)
Net other comprehensive loss (income)			1,897	(1,260)
Ending Balance	$ 9,608	$ 7,901	$ 9,608	$ 7,901