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							9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income (loss)	$ 108,066	$ 116,143	$ 107,737	$ 150,730	$ 124,871	$ 52,926	$ (1,851,947)	$ 331,946	$ (1,674,150)
Weighted average number of common shares outstanding - basic (in shares)	218,416				220,221			219,791	220,216
Effect of potentially dilutive common shares (in shares)	562				197			487	0
Weighted average number of shares outstanding, diluted (in shares)	218,978				220,418			220,278	220,216
Earnings (loss) per common share:
Basic (in dollars per share)	$ 0.49				$ 0.57			$ 1.51	$ (7.60)
Diluted (in dollars per share)	$ 0.49				$ 0.57			$ 1.51	$ (7.60)