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						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income used for calculating basic and diluted income per common share	$ 16,964	$ 55,077	$ 75,097	$ 41,388	$ 103,406	$ 20,795	$ 147,138	$ 165,589
Denominator:
Weighted average number of common shares used in the calculation of basic income per share (in shares)	89,378			89,846			89,307	89,947
Common stock equivalents associated with stock-based compensation plans (in shares)	577			1,041			695	1,142
Shares used in the calculation of diluted income per share (in shares)	89,955			90,887			90,002	91,089
Basic income per share (in dollars per share)	$ 0.19			$ 0.46			$ 1.65	$ 1.84
Diluted income per share (in dollars per share)	$ 0.19			$ 0.46			$ 1.63	$ 1.82