Condensed Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balances at Dec. 31, 2017	$ 249,172	$ 30	$ 508,228	$ (257,844)	$ (1,242)
Beginning balances, shares at Dec. 31, 2017		29,737,561
Adoption of accounting standard	967			967
Exercise of common stock options	1,941		1,941
Exercise of common stock options, shares		135,874
Issuance of common stock upon release of restricted stock units		73,352
Shares withheld for tax obligations	(215)		(215)
Shares withheld for tax obligations, shares		(3,019)
Stock based compensation	8,244		8,244
Net loss	(17,713)			(17,713)
Other comprehensive loss	226				226
Ending balances at Mar. 31, 2018	242,622	$ 30	518,198	(274,590)	(1,016)
Ending balances, shares at Mar. 31, 2018		29,943,768
Beginning balances at Dec. 31, 2017	249,172	$ 30	508,228	(257,844)	(1,242)
Beginning balances, shares at Dec. 31, 2017		29,737,561
Net loss	(39,598)
Other comprehensive loss	295
Ending balances at Sep. 30, 2018	244,302	$ 30	541,694	(296,475)	(947)
Ending balances, shares at Sep. 30, 2018		30,150,577
Beginning balances at Dec. 31, 2017	249,172	$ 30	508,228	(257,844)	(1,242)
Beginning balances, shares at Dec. 31, 2017		29,737,561
Net loss	(49,200)
Ending balances at Dec. 31, 2018	245,483	$ 30	552,612	(306,082)	(1,077)
Ending balances, shares at Dec. 31, 2018		30,263,536
Beginning balances at Mar. 31, 2018	242,622	$ 30	518,198	(274,590)	(1,016)
Beginning balances, shares at Mar. 31, 2018		29,943,768
Exercise of common stock options	1,465		1,465
Exercise of common stock options, shares		63,351
Issuance of common stock upon release of restricted stock units		42,938
Shares withheld for tax obligations	(727)		(727)
Shares withheld for tax obligations, shares		(9,250)
Issuance of common stock under employee stock purchase plan	3,580		3,580
Issuance of common stock under employee stock purchase plan, shares		53,824
Stock based compensation	9,314		9,314
Net loss	(10,620)			(10,620)
Other comprehensive loss	(45)				(45)
Ending balances at Jun. 30, 2018	245,589	$ 30	531,830	(285,210)	(1,061)
Ending balances, shares at Jun. 30, 2018		30,094,631
Exercise of common stock options	494		494
Exercise of common stock options, shares		43,178
Issuance of common stock upon release of restricted stock units		14,060
Shares withheld for tax obligations	(83)		(83)
Shares withheld for tax obligations, shares		(1,292)
Stock based compensation	9,453		9,453
Net loss	(11,265)			(11,265)
Other comprehensive loss	114				114
Ending balances at Sep. 30, 2018	244,302	$ 30	541,694	(296,475)	(947)
Ending balances, shares at Sep. 30, 2018		30,150,577
Beginning balances at Dec. 31, 2018	245,483	$ 30	552,612	(306,082)	(1,077)
Beginning balances, shares at Dec. 31, 2018		30,263,536
Exercise of common stock options	1,928		1,928
Exercise of common stock options, shares		105,462
Issuance of common stock upon release of restricted stock units		83,008
Shares withheld for tax obligations	(154)		(154)
Shares withheld for tax obligations, shares		(4,237)
Stock based compensation	10,397		10,397
Net loss	(44,076)			(44,076)
Other comprehensive loss	521				521
Ending balances at Mar. 31, 2019	214,099	$ 30	564,783	(350,158)	(556)
Ending balances, shares at Mar. 31, 2019		30,447,769
Beginning balances at Dec. 31, 2018	245,483	$ 30	552,612	(306,082)	(1,077)
Beginning balances, shares at Dec. 31, 2018		30,263,536
Net loss	(89,944)
Other comprehensive loss	539
Ending balances at Sep. 30, 2019	198,097	$ 31	594,630	(396,026)	(538)
Ending balances, shares at Sep. 30, 2019		30,994,005
Beginning balances at Mar. 31, 2019	214,099	$ 30	564,783	(350,158)	(556)
Beginning balances, shares at Mar. 31, 2019		30,447,769
Exercise of common stock options	2,233	$ 1	2,232
Exercise of common stock options, shares		214,647
Issuance of common stock upon release of restricted stock units		95,347
Shares withheld for tax obligations	(406)		(406)
Shares withheld for tax obligations, shares		(6,530)
Issuance of common stock under employee stock purchase plan	4,095		4,095
Issuance of common stock under employee stock purchase plan, shares		119,393
Stock based compensation	9,724		9,724
Net loss	(28,021)			(28,021)
Other comprehensive loss	(182)				(182)
Ending balances at Jun. 30, 2019	201,542	$ 31	580,428	(378,179)	(738)
Ending balances, shares at Jun. 30, 2019		30,870,626
Exercise of common stock options	3,505		3,505
Exercise of common stock options, shares		83,346
Issuance of common stock upon release of restricted stock units		46,228
Shares withheld for tax obligations	(497)		(497)
Shares withheld for tax obligations, shares		(6,195)
Stock based compensation	11,194		11,194
Net loss	(17,847)			(17,847)
Other comprehensive loss	200				200
Ending balances at Sep. 30, 2019	$ 198,097	$ 31	$ 594,630	$ (396,026)	$ (538)
Ending balances, shares at Sep. 30, 2019		30,994,005