Net Income Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Income Per Common Share
Number of outstanding units not included in computation of Diluted EPS (in shares)	241,000	372,600
Numerator
Net Income	$ 88,181	$ 54,362
Denominator
Weighted average number of common shares outstanding	51,440,000	51,921,000
Effect of outstanding share-based awards	658,000	1,112,000
Weighted average number of shares for diluted net income per common share	52,098,000	53,033,000
Basic earnings per share (in dollars per share)	$ 1.71	$ 1.05
Diluted earnings per share (in dollars per share)	$ 1.69	$ 1.03