Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ 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
Net Income Per Common Share
Number of outstanding units not included in computation of Diluted EPS (in shares)							200,000	150,000
Numerator:
Net income	$ 33,662	$ (25,715)	$ 29,988	$ 91,339	$ 88,052	$ 88,181	$ 37,935	$ 267,572
Denominator:
Weighted average number of common shares outstanding	50,181,000			50,746,000			50,199,000	51,111,000
Effect of outstanding share-based awards	441,000			383,000			246,000	457,000
Diluted earnings per share weighted average shares	50,622,000			51,129,000			50,445,000	51,568,000
Basic earnings per share (in dollars per share)	$ 0.67			$ 1.80			$ 0.76	$ 5.24
Diluted earnings per share (in dollars per share)	$ 0.66			$ 1.79			$ 0.75	$ 5.19