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		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
NUMERATORS:
Net income	$ 61,333	$ 61,809	$ 164,148	$ 163,665	$ 232,940
Dividends and undistributed earnings allocated to participating securities	14	31	40	92
Net earnings available to common shareholders	$ 61,319	$ 61,778	$ 164,108	$ 163,573
DENOMINATORS:
Weighted average number of common shares outstanding - basic (in shares)	220,215	220,291	220,270	220,313
Effect of potentially dilutive common shares (in shares)	540	460	523	623
Weighted average number of common shares outstanding - diluted (in shares)	220,755	220,751	220,793	220,936
EARNINGS PER COMMON SHARE:
Basic (in usd per share)	$ 0.28	$ 0.28	$ 0.75	$ 0.74
Diluted (in usd per share)	$ 0.28	$ 0.28	$ 0.74	$ 0.74