Computation of Earnings Per Share - Calculations Of Basic And Diluted (Loss) Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income used for calculating basic and diluted income per common share	$ 103,406	$ 46,270	$ 124,201	$ 26,683
Denominator:
Weighted average number of common shares used in the calculation of basic (loss) income per share (in shares)	90,112	91,048	89,997	91,026
Common stock equivalents associated with stock-based compensation plans (in shares)	1,076	1,108	1,149	1,226
Shares used in the calculation of diluted income per share (in shares)	91,188	92,156	91,146	92,252
Basic income per share (in dollars per share)	$ 1.15	$ 0.51	$ 1.38	$ 0.29
Diluted income per share (in dollars per share)	$ 1.13	$ 0.50	$ 1.36	$ 0.29