Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Income Per Common Share
Number of outstanding units not included in computation of Diluted EPS (in shares)			165,000	207,000
Numerator
Net Income	$ 88,052	$ 75,859	$ 176,233	$ 130,220
Denominator
Weighted average number of common shares outstanding	51,145,000	52,046,000	51,293,000	51,983,000
Effect of outstanding share-based awards	332,000	867,000	494,000	990,000
Weighted average number of shares for diluted net income per common share	51,477,000	52,913,000	51,787,000	52,973,000
Basic earnings per share (in dollars per share)	$ 1.72	$ 1.46	$ 3.44	$ 2.51
Diluted earnings per share (in dollars per share)	$ 1.71	$ 1.43	$ 3.40	$ 2.46