Earnings (Loss) Per Common Share (Computation Of Basic And Diluted Earnings Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net income (loss)	$ 116,143	$ 107,737	$ 150,730	$ 124,871	$ 52,926	$ (1,851,947)	$ 223,880	$ (1,799,021)
Weighted average number of common shares outstanding - basic (in shares)	220,593				220,210		220,481	220,213
Effect of potentially dilutive common shares (in shares)	429				110		447	0
Weighted average number of shares outstanding, diluted (in shares)	221,022				220,320		220,928	220,213
Earnings (loss) per common share:
Basic (in dollars per share)	$ 0.53				$ 0.24		$ 1.02	$ (8.17)
Diluted (in dollars per share)	$ 0.53				$ 0.24		$ 1.01	$ (8.17)