Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Total
Beginning balance at Dec. 31, 2017	$ 295	$ 566,790	$ 168,148	$ 735,233
Beginning balance (in shares) at Dec. 31, 2017	29,503
Issuance of common stock	$ 6	14,302		14,308
Issuance of common stock, shares	776
Repurchase of common stock		(4,788)		(4,788)
Repurchase of common stock, shares	(160)
Stock-based compensation expense		6,323		6,323
Net income			53,212	53,212
Ending balance at Jun. 30, 2018	$ 301	582,627	220,778	803,706
Ending balance (in shares) at Jun. 30, 2018	30,119
Beginning balance at Mar. 31, 2018	$ 298	569,536	187,584	757,418
Beginning balance (in shares) at Mar. 31, 2018	29,790
Issuance of common stock	$ 3	9,284		9,287
Issuance of common stock, shares	320
Vesting of restricted stock units			1	1
Vesting of restricted stock units, shares	9
Stock-based compensation expense		3,807		3,807
Net income			33,193	33,193
Ending balance at Jun. 30, 2018	$ 301	582,627	220,778	803,706
Ending balance (in shares) at Jun. 30, 2018	30,119
Adoption of ASC 606			(582)	(582)
Beginning balance at Dec. 31, 2018	$ 302	595,037	264,020	859,359
Beginning balance (in shares) at Dec. 31, 2018	30,155
Issuance of common stock	$ 1	2,662		2,663
Issuance of common stock, shares	100
Repurchase of common stock	$ (1)	(1,438)		(1,439)
Repurchase of common stock, shares	(83)
Vesting of restricted stock units	$ 2	(3,436)		(3,434)
Vesting of restricted stock units, shares	267
Stock-based compensation expense		7,468		7,468
Net income			32,612	32,612
Ending balance at Jun. 30, 2019	$ 304	600,293	296,632	897,229
Ending balance (in shares) at Jun. 30, 2019	30,439
Beginning balance at Mar. 31, 2019	$ 303	593,966	281,137	875,406
Beginning balance (in shares) at Mar. 31, 2019	30,304
Issuance of common stock	$ 1	2,662		2,663
Issuance of common stock, shares	100
Vesting of restricted stock units		(268)		(268)
Vesting of restricted stock units, shares	35
Stock-based compensation expense		3,933		3,933
Net income			15,495	15,495
Ending balance at Jun. 30, 2019	$ 304	$ 600,293	$ 296,632	$ 897,229
Ending balance (in shares) at Jun. 30, 2019	30,439