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
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
NUMERATORS:
Net income	$ 90,981	$ 65,999	$ 78,972	$ 75,251	$ 63,807	$ 57,219	$ 46,036	$ 235,952	$ 167,062
Dividends and undistributed earnings allocated to participating securities	5				14			15	40
Net earnings available to common shareholders	$ 90,976				$ 63,793			$ 235,937	$ 167,022
DENOMINATORS:
Weighted average number of common shares outstanding - basic (in shares)	220,224				220,215			220,292	220,270
Effect of potentially dilutive common shares (in shares)	396				540			459	523
Weighted average number of common shares outstanding - diluted (in shares)	220,620				220,755			220,751	220,793
EARNINGS PER COMMON SHARE:
Basic (in usd per share)	$ 0.41				$ 0.29			$ 1.07	$ 0.76
Diluted (in usd per share)	$ 0.41				$ 0.29			$ 1.07	$ 0.76