Earnings Per Share (Schedule Of Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share [Abstract]
Net Income (Loss) Attributable to Parent	$ 521,082	$ 272,014	$ 57,767	$ (45,468)	$ 53,165	$ 74,497	$ (409,368)	$ 21,253	$ 805,395	$ (260,453)	$ 740,101
Participating securities share in earnings									5,047	1,003	2,306
Net income (loss) attributable to common shares									$ 800,348	$ (261,456)	$ 737,795
Average number of shares of common stock outstanding	176,265	176,149	176,147	176,210	175,936	175,871	175,865	176,737	176,174	176,101	188,731
Effect of dilutive variable restricted shares and performance share units									1,022	0	209
Average number of shares of common stock outstanding assuming dilution	177,457	176,530	176,302	176,210	176,137	175,993	175,865	176,784	177,196	176,101	188,940
Basic earnings (loss) per share	$ 2.94	$ 1.53	$ 0.33	$ (0.26)	$ 0.30	$ 0.42	$ (2.33)	$ 0.12	$ 4.54	$ (1.48)	$ 3.91
Diluted earnings (loss) per share	$ 2.92	$ 1.53	$ 0.33	$ (0.26)	$ 0.30	$ 0.42	$ (2.33)	$ 0.12	$ 4.52	$ (1.48)	$ 3.90
Excludes anti-dilutive restricted and performance share units									543	469	89