Selected Quarterly Financial Data (Schedule Of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Financial Data [Abstract]
Revenues	$ 135,462	$ 109,318	$ 155,477	$ 115,618	$ 138,457	$ 102,217	$ 141,835	$ 98,588	$ 515,875	$ 481,097	$ 467,403
Operating income	50,424	28,602	62,795	33,019	41,170	15,903	51,773	19,081	174,840	127,927	136,959
Net income	47,372	21,247	45,846	24,110	31,949	9,643	37,536	13,435	138,575	92,563	94,445
Net income	$ 46,529	$ 20,825	$ 45,083	$ 23,777	$ 31,417	$ 9,382	$ 37,008	$ 13,174	$ 136,214	$ 90,981	$ 93,644
Basic earnings (loss) per share	$ 0.43	$ 0.19	$ 0.42	$ 0.22	$ 0.29	$ 0.09	$ 0.35	$ 0.12	$ 1.27	$ 0.85	$ 0.88
Diluted earnings (loss) per share	$ 0.43	$ 0.19	$ 0.42	$ 0.22	$ 0.29	$ 0.09	$ 0.34	$ 0.12	$ 1.27	$ 0.85	$ 0.87