Quarterly Financial Data (Schedule Of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Data [Abstract]
Revenues	$ 443,073	$ 433,945	$ 427,294	$ 423,362	$ 416,505	$ 411,319	$ 405,532	$ 399,769	$ 1,727,674	$ 1,633,125	$ 1,638,474
Operating income	119,081	117,011	114,590	107,819	107,430	108,210	74,066	97,602	458,501	387,308	312,456
Depreciation, accretion, and amortization	(162,643)	(161,907)	(159,520)	(159,030)	(158,554)	(160,111)	(159,723)	(159,801)	(643,100)	(638,189)	(660,021)
Loss from extinguishment of debt, net			(1,961)		(18,189)	(34,512)			(1,961)	(52,701)	(783)
Net income (loss)	$ 7,660	$ 49,161	$ 9,233	$ 37,600	$ 5,256	$ (15,370)	$ 32,711	$ 53,641	$ 103,654	$ 76,238	$ (175,656)
Net income per common share - basic	$ 0.07	$ 0.41	$ 0.08	$ 0.31	$ 0.04	$ (0.12)	$ 0.26	$ 0.43	$ 0.86	$ 0.61	$ (1.37)
Net income per common share - diluted	$ 0.06	$ 0.41	$ 0.08	$ 0.31	$ 0.04	$ (0.12)	$ 0.26	$ 0.43	$ 0.86	$ 0.61	$ (1.37)