Summary Of Significant Accounting Policies (Schedule Of Calculation 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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Reconciliation [Line Items]
Net income (loss)	$ (76,340)	$ 17,512	$ 17,176	$ 9,687	$ 18,568	$ 21,742	$ 12,685	$ 11,052	$ 39,421	$ 20,565	$ 21,210	$ 14,205	$ (31,965)	$ 64,047	$ 95,401
Denominator for basic earnings per share - weighted-average number of common shares outstanding during the period													49,680	49,298	51,573
Denominator for diluted earnings per share													49,680	49,825	52,088
Basic earnings per share	$ (1.54)	$ 0.35	$ 0.35	$ 0.2	$ 0.38	$ 0.44	$ 0.26	$ 0.22	$ 0.78	$ 0.4	$ 0.41	$ 0.27	$ (0.64)	$ 1.3	$ 1.85
Diluted earnings per share	$ (1.54)	$ 0.35	$ 0.34	$ 0.19	$ 0.37	$ 0.44	$ 0.26	$ 0.22	$ 0.77	$ 0.4	$ 0.4	$ 0.27	$ (0.64)	$ 1.29	$ 1.83
Stock Options [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to stock-based awards													0	313	318
Restricted Stock Units [Member]
Earnings Per Share Reconciliation [Line Items]
Incremental common shares attributable to stock-based awards													0	214	197