Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Income per share
Strike price of the warrant transactions (in dollars per share)						$ 80.9063
Numerator, in thousands:
Net earnings	$ 28,860	$ 32,727	$ 18,340	$ 28,011	$ 30,737	$ 26,352	$ 79,927	$ 85,100
Denominator:
Basic (in shares)	52,453,384			52,227,630			52,392,232	51,897,240
Effect of dilutive securities:
Stock options and SAR (in shares)	1,352,454			2,012,217			1,506,254	2,201,090
Diluted (in shares)	53,805,838			54,239,847			53,898,486	54,098,330
Basic (in dollars per share)	$ 0.55			$ 0.54			$ 1.53	$ 1.64
Diluted (in dollars per share)	$ 0.54			$ 0.52			$ 1.48	$ 1.57
Stock options
Income per share
Common stock equivalents excluded in the calculation of diluted income per share (in shares)	1,395,138			165,675			961,605	180,100
2023 Notes
Income per share
Conversion price, per share of common stock (in dollars per share)						$ 59.33