Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Income per share
Strike price of the warrant transactions (in dollars per share)				$ 80.9063
Numerator, in thousands:
Net earnings used for calculation of basic and diluted EPS	$ 32,727	$ 18,340	$ 30,737	$ 26,352	$ 51,067	$ 57,089
Denominator:
Basic (in shares)	52,385,590		51,919,894		52,361,149	51,729,243
Effect of dilutive potential common shares:
Stock options and SAR (in shares)	1,527,387		2,283,414		1,586,685	2,292,698
Diluted (in shares)	53,912,977		54,203,308		53,947,834	54,021,941
Basic (in dollars per share)	$ 0.62		$ 0.59		$ 0.98	$ 1.10
Diluted (in dollars per share)	$ 0.61		$ 0.57		$ 0.95	$ 1.06
Stock options
Income per share
Common stock equivalents excluded in the calculation of diluted income per share (in shares)	1,030,370		137,565		300,342	184,760
2023 Notes
Income per share
Conversion price, per share of common stock (in dollars per share)				$ 59.33