Summary of Significant Accounting Policies (Schedule Of Basic and Diluted Earnings Per 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
Accounting Policies [Abstract]
Net Income	$ 141,134	$ 22,517	$ 23,219	$ 16,019	$ 21,811	$ 18,920	$ 18,306	$ 20,092	$ 202,889	$ 79,129	$ 123,714
Weighted average common shares outstanding									72,270	72,057	71,957
Dilutive effect of stock-based awards									288	336	599
Shares used in computing diluted earnings per share									72,558	72,393	72,556
Basic earnings per share	$ 1.95	$ 0.31	$ 0.32	$ 0.22	$ 0.30	$ 0.26	$ 0.25	$ 0.28	$ 2.81	$ 1.10	$ 1.72
Diluted earnings per share	$ 1.94	$ 0.31	$ 0.32	$ 0.22	$ 0.30	$ 0.26	$ 0.25	$ 0.28	$ 2.80	$ 1.09	$ 1.71
Number of antidilutive options that were excluded from computation of earnings per share									0	0	0