CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in- Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Balance at Dec. 31, 2016	$ 185,448	$ 63,716	$ 8,243	$ 27	$ 113,462
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85,718				85,718
Other comprehensive income, net of tax	39			39
Issuance of 2,025,000 shares in public offering, net of expenses	93,306	93,306
Issuance of shares upon exercise of common stock options	926	926
Shares used to pay taxes on stock grants	(5,163)	(5,163)
Stock-based compensation expense	10,411	10,411
Other comprehensive loss, net of tax	39
Balance at Dec. 31, 2017	370,685	163,196	8,243	66	199,180
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	119,832				119,832
Issuance of shares upon exercise of common stock options	3	3
Shares used to pay taxes on stock grants	(2,961)	(2,961)
Stock-based compensation expense	13,981	13,981
Other comprehensive loss, net of tax	(2,746)			(2,746)
Stock repurchases under buyback program	(107,567)	(12,783)	(646)		(94,138)
Purchase of convertible notes hedges	31,481		31,481
Proceeds from sale of warrants	18,147		18,147
Equity component of convertible note issuance	30,861		30,861
Balance at Dec. 31, 2018	408,754	161,436	25,124	(2,680)	224,874
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	89,566				89,566
Shares used to pay taxes on stock grants	(3,511)	(3,511)
Stock-based compensation expense	15,436	15,436	0
Other comprehensive loss, net of tax	(3,018)			(3,018)
Dividends declared	(5,938)				(5,938)
Issuance of shares upon exercise of common stock options	7	7
Stock repurchases under buyback program	(3,815)	(706)	(110)		(2,999)
Balance at Dec. 31, 2019	$ 497,481	$ 172,662	$ 25,014	$ (5,698)	$ 305,503