Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 15, 2018	Mar. 14, 2018
Income per share
Strike price of the warrant transactions (in dollars per share)								$ 80.9063
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net earnings									$ 113,056	$ 110,993	$ 57,284
Interest expense on Convertible Senior Secured Notes due 2019									0	0	134
Changes in fair value of derivative liabilities									0	0	(76)
Loss on extinguishment of debt									0	0	295
Loss on extinguishment of outstanding debt, as if converted									0	0	(321)
Total adjustments									0	0	32
Net earnings used for calculation of diluted EPS									$ 113,056	$ 110,993	$ 57,316
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Weighted average shares outstanding, basic (in shares)									52,412,181	51,989,824	50,756,603
Incremental Weighted Average Shares Attributable to Dilutive Effect [Abstract]
Shares underlying Convertible Senior Notes (in shares)									0	0	285,257
Shares issuable to settle interest make-whole derivatives (in shares)									0	0	7,012
Stock options and SAR (in shares)									1,404,573	2,109,048	2,252,278
Total dilutive potential common shares (in shares)									1,404,573	2,109,048	2,544,547
Weighted average shares outstanding, diluted (in shares)									53,816,754	54,098,872	53,301,150
Earnings per share, basic (in dollars per share)	$ 0.63	$ 0.55	$ 0.62	$ 0.35	$ 0.50	$ 0.54	$ 0.59	0.51	$ 2.16	$ 2.13	$ 1.13
Earnings per share, diluted (in dollars per share)	$ 0.62	$ 0.54	$ 0.61	$ 0.34	$ 0.48	$ 0.52	$ 0.57	$ 0.49	$ 2.10	$ 2.05	$ 1.08
Stock options (in shares)
Income per share
Common stock equivalents excluded in the calculation of diluted income per share (in shares)									1,145,446	199,982	40,009
2023 Notes
Income per share
Conversion price, per share of common stock												$ 59.33	$ 59.33