Computation of Earnings Per Share (Details) - Calculations Of Basic And Diluted Earnings Per Share - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income used for calculating basic and diluted earnings per common share	$ 3,279	$ 193,582	$ 175,635
Denominator:
Weighted average number of common shares used in the calculation of basic earnings per share	88,466	82,571	83,852
Common share equivalents associated with stock-based compensation plans	1,324	1,249	1,204
Shares used in the calculation of diluted earnings per share	89,790	83,820	85,056
Earnings per share: (2)
Basic (in Dollars per share)	$ 0.04	$ 2.34	$ 2.09
Diluted (in Dollars per share)	$ 0.04	$ 2.31	$ 2.06