Earnings Per Common Share (Computation Of Basic And Diluted Earnings (Loss) Per Common Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
NUMERATORS:
Net income	$ 77,669	$ 46,003	$ 246,019
Dividends and undistributed earnings allocated to participating securities	6	12
Net earnings available to common shareholders	$ 77,663	$ 45,991
DENOMINATORS:
Weighted average number of common shares outstanding - basic (in shares)	220,370	220,287
Effect of potentially dilutive common shares (in shares)	455	492
Weighted average number of common shares outstanding - diluted (in shares)	220,825	220,779
EARNINGS PER COMMON SHARE:
Basic (in usd per share)	$ 0.35	$ 0.21
Diluted (in usd per share)	$ 0.35	$ 0.21