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
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NUMERATORS:
Net income	$ 81,871	$ 61,333	$ 56,812	$ 46,003	$ 69,275	$ 61,809	$ 54,287	$ 47,569	$ 246,019	$ 232,940	$ 222,539
Dividends and undistributed earnings allocated to participating securities	16	14	14	12	33	31	32	29	56	125	357
Net earnings available to common shareholders	$ 81,855	$ 61,319	$ 56,798	$ 45,991	$ 69,242	$ 61,778	$ 54,255	$ 47,540	$ 245,963	$ 232,815	$ 222,182
DENOMINATORS:
Weighted average number of common shares outstanding - basic									220,251	220,282	220,327
Effect of potentially dilutive common shares (2)									585	626	718
Weighted average number of common shares outstanding - diluted									220,836	220,908	221,045
EARNINGS PER COMMON SHARE:
Basic earnings per common share (usd per share)	$ 0.37	$ 0.28	$ 0.26	$ 0.21	$ 0.31	$ 0.28	$ 0.25	$ 0.22	$ 1.12	$ 1.06	$ 1.01
Diluted earnings per common share (usd per share)	$ 0.37	$ 0.28	$ 0.26	$ 0.21	$ 0.31	$ 0.28	$ 0.25	$ 0.22	$ 1.11	$ 1.05	$ 1.01