Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid In Capital [Member]	(Accumulated deficit) Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Jan. 01, 2017	$ 672,909	$ 140	$ 597,269	$ 75,500
Beginning Balance, Shares at Jan. 01, 2017		140,002,242
Net income	158,440			158,440
Other comprehensive income (loss)	(784)				$ (784)
Issuance of shares under stock plans	9,300	$ 2	9,298
Issuance of shares under stock plans, Shares		2,144,669
Repurchase and retirement of common stock	(203,392)	$ (10)		(203,382)
Repurchase and retirement of common stock, Shares		(9,696,819)
Equity-based compensation	14,221		14,221
Ending Balance at Dec. 31, 2017	650,694	$ 132	620,788	30,558	(784)
Ending Balance, Shares at Dec. 31, 2017		132,450,092
Net income	145,833			145,833
Other comprehensive income (loss)	5,351				5,351
Issuance of shares under stock plans	21,051	$ 3	21,048
Issuance of shares under stock plans, Shares		3,170,818
Repurchase and retirement of common stock	$ (193,307)	$ (8)		(193,299)
Repurchase and retirement of common stock, Shares	(8,411,575)	(8,411,575)
Equity-based compensation	$ 11,673		11,673
Ending Balance at Sep. 30, 2018	$ 641,295	$ 127	$ 653,509	$ (16,908)	$ 4,567
Ending Balance, Shares at Sep. 30, 2018		127,209,335