Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2017	$ 343,544	$ 39	$ 304,155	$ (574)	$ 39,924
Balance (in shares) at Dec. 31, 2017		38,598,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	9,142				9,142
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(391)			(391)
Issuance of common stock upon exercise of stock options	7,933		7,933
Issuance of common stock upon exercise of stock options (shares)		285,997
Stock Repurchased During Period, Shares		(21,288)
Treasury Stock, Value, Acquired, Cost Method	(1,481)		(255)		(1,226)
Adjustment To Additional Paid In Capital, Share-Based Compensation, Taxes Related To Net Share Settlements Of Equity Awards		(63,695)
Issuance of common stock upon vesting of restricted stock units (in shares)		158,561
Taxes from release of the restricted share units	(4,030)		(4,030)
Stock-based compensation	8,891		8,891
Ending balance at Mar. 31, 2018	366,319	$ 39	316,694	(965)	50,551
Balance (in shares) at Mar. 31, 2018		38,957,692
Beginning balance at Dec. 31, 2017	343,544	$ 39	304,155	(574)	39,924
Balance (in shares) at Dec. 31, 2017		38,598,117
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,904
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(178)
Change in unrealized loss on investments	0
Ending balance at Sep. 30, 2018	374,140	$ 39	329,045	(752)	45,808
Balance (in shares) at Sep. 30, 2018		39,341,489
Beginning balance at Mar. 31, 2018	366,319	$ 39	316,694	(965)	50,551
Balance (in shares) at Mar. 31, 2018		38,957,692
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	10,293				10,293
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	144			144
Issuance of common stock upon exercise of stock options	4,239		4,239
Issuance of common stock upon exercise of stock options (shares)		144,851
Stock Repurchased During Period, Shares		(235,539)
Treasury Stock, Value, Acquired, Cost Method	(17,875)		(2,826)		(15,049)
Adjustment To Additional Paid In Capital, Share-Based Compensation, Taxes Related To Net Share Settlements Of Equity Awards		(65,214)
Issuance of common stock upon vesting of restricted stock units (in shares)		184,008
Taxes from release of the restricted share units	(4,904)		(4,904)
Stock-based compensation	7,023		7,023
Ending balance at Jun. 30, 2018	365,239	$ 39	320,226	(821)	45,795
Balance (in shares) at Jun. 30, 2018		38,985,798
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,469				23,469
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	69			69
Change in unrealized loss on investments	0
Issuance of common stock upon exercise of stock options	8,721		8,721
Issuance of common stock upon exercise of stock options (shares)		611,431
Stock Repurchased During Period, Shares		(310,815)
Treasury Stock, Value, Acquired, Cost Method	(27,185)		(3,729)		(23,456)
Adjustment To Additional Paid In Capital, Share-Based Compensation, Taxes Related To Net Share Settlements Of Equity Awards		(35,881)
Issuance of common stock upon vesting of restricted stock units (in shares)		90,956
Taxes from release of the restricted share units	(3,075)		(3,075)
Stock-based compensation	6,902		6,902
Ending balance at Sep. 30, 2018	374,140	$ 39	329,045	(752)	45,808
Balance (in shares) at Sep. 30, 2018		39,341,489
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Retained Earnings (Accumulated Deficit)	27,964
Beginning balance at Dec. 31, 2018	357,989	$ 39	330,572	(586)	27,964
Balance (in shares) at Dec. 31, 2018		39,015,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	13,266				13,266
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	896			896
Issuance of common stock upon exercise of stock options	4,047		4,047
Issuance of common stock upon exercise of stock options (shares)		152,164
Stock Repurchased During Period, Shares		(94,090)
Treasury Stock, Value, Acquired, Cost Method	(7,871)		(1,129)		(6,742)
Adjustment To Additional Paid In Capital, Share-Based Compensation, Taxes Related To Net Share Settlements Of Equity Awards		(38,877)
Issuance of common stock upon vesting of restricted stock units (in shares)		99,601
Taxes from release of the restricted share units	(3,367)		(3,367)
Stock-based compensation	8,443		8,443
Ending balance at Mar. 31, 2019	373,403	$ 39	338,566	310	34,488
Balance (in shares) at Mar. 31, 2019		39,133,832
Beginning balance at Dec. 31, 2018	357,989	$ 39	330,572	(586)	27,964
Balance (in shares) at Dec. 31, 2018		39,015,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	48,672
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	2,857
Change in unrealized loss on investments	$ 1,413
Issuance of common stock upon exercise of stock options (shares)	412,910
Ending balance at Sep. 30, 2019	$ 361,023	$ 39	345,378	2,271	13,335
Balance (in shares) at Sep. 30, 2019		38,750,803
Beginning balance at Mar. 31, 2019	373,403	$ 39	338,566	310	34,488
Balance (in shares) at Mar. 31, 2019		39,133,832
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,232				16,232
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	930			930
Issuance of common stock upon exercise of stock options	4,944		4,944
Issuance of common stock upon exercise of stock options (shares)		192,687
Stock Repurchased During Period, Shares		(183,948)
Treasury Stock, Value, Acquired, Cost Method	(16,245)		(2,207)		(14,038)
Adjustment To Additional Paid In Capital, Share-Based Compensation, Taxes Related To Net Share Settlements Of Equity Awards		(45,250)
Issuance of common stock upon vesting of restricted stock units (in shares)		126,754
Taxes from release of the restricted share units	(4,044)		(4,044)
Stock-based compensation	8,378		8,378
Ending balance at Jun. 30, 2019	383,598	$ 39	345,637	1,240	36,682
Balance (in shares) at Jun. 30, 2019		39,224,075
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	19,174				19,174
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1,031			1,031
Change in unrealized loss on investments	1,009
Issuance of common stock upon exercise of stock options	2,023		2,023
Issuance of common stock upon exercise of stock options (shares)		68,059
Stock Repurchased During Period, Shares		(603,417)
Treasury Stock, Value, Acquired, Cost Method	(49,761)		(7,240)		(42,521)
Adjustment To Additional Paid In Capital, Share-Based Compensation, Taxes Related To Net Share Settlements Of Equity Awards		(41,120)
Issuance of common stock upon vesting of restricted stock units (in shares)		103,206
Taxes from release of the restricted share units	(3,453)		(3,453)
Stock-based compensation	8,411		8,411
Ending balance at Sep. 30, 2019	361,023	$ 39	$ 345,378	$ 2,271	$ 13,335
Balance (in shares) at Sep. 30, 2019		38,750,803
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Retained Earnings (Accumulated Deficit)	$ 13,335