Summary of Significant Accounting Policies (Schedule Of Common Stock And Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accounting Policies [Abstract]
Net Income	$ 25,981	$ 25,128	$ 30,680	$ 21,245	$ 29,368	$ 29,578	$ 27,518	$ 16,293	$ 103,034	$ 102,757	$ 80,039
Weighted average common shares outstanding									72,909	72,787	72,369
Dilutive effect of stock-based awards									544	438	438
Shares used in computing diluted earnings per share									73,453	73,225	72,807
Basic earnings per share	$ 0.36	$ 0.34	$ 0.42	$ 0.29	$ 0.40	$ 0.41	$ 0.38	$ 0.22	$ 1.41	$ 1.41	$ 1.11
Diluted earnings per share	$ 0.35	$ 0.34	$ 0.42	$ 0.29	$ 0.40	$ 0.40	$ 0.38	$ 0.22	$ 1.40	$ 1.40	$ 1.10
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0	0	0