Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	APIC	Accumulated Deficit
Increase (Decrease) in Stockholders' Equity
Cumulative-effect of accounting change adopted	$ 5,923			$ 5,923
Beginning Balance at Dec. 31, 2017	313,118	$ 10	$ 664,192	(351,084)
Balance (in shares) at Dec. 31, 2017		100,428,656
Increase (Decrease) in Stockholders' Equity
Net loss	(9,055)			(9,055)
Stock-based compensation	9,431		9,431
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	(166)		(166)
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		468,809
Ending Balance at Mar. 31, 2018	319,251	$ 10	673,457	(354,216)
Balance (in shares) at Mar. 31, 2018		100,897,465
Beginning Balance at Dec. 31, 2017	313,118	$ 10	664,192	(351,084)
Balance (in shares) at Dec. 31, 2017		100,428,656
Increase (Decrease) in Stockholders' Equity
Net loss	(21,928)
Ending Balance at Sep. 30, 2018	343,842	$ 10	710,921	(367,089)
Balance (in shares) at Sep. 30, 2018		103,819,348
Beginning Balance at Mar. 31, 2018	319,251	$ 10	673,457	(354,216)
Balance (in shares) at Mar. 31, 2018		100,897,465
Increase (Decrease) in Stockholders' Equity
Net loss	(6,622)			(6,622)
Stock-based compensation	9,445		9,445
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	1,905		1,905
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		708,935
Ending Balance at Jun. 30, 2018	323,979	$ 10	684,807	(360,838)
Balance (in shares) at Jun. 30, 2018		101,606,400
Increase (Decrease) in Stockholders' Equity
Net loss	(6,251)			(6,251)
Stock-based compensation	10,798		10,798
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	15,316		15,316
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		2,212,948
Ending Balance at Sep. 30, 2018	343,842	$ 10	710,921	(367,089)
Balance (in shares) at Sep. 30, 2018		103,819,348
Increase (Decrease) in Stockholders' Equity
Cumulative-effect of accounting change adopted	(3,690)			(3,690)
Beginning Balance at Dec. 31, 2018	$ 346,553	$ 10	720,025	(373,482)
Balance (in shares) at Dec. 31, 2018	104,337,508	104,337,508
Increase (Decrease) in Stockholders' Equity
Net loss	$ (14,365)			(14,365)
Stock-based compensation	9,108		9,108
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	2,261		2,261
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		781,538
Ending Balance at Mar. 31, 2019	339,867	$ 10	731,394	(391,537)
Balance (in shares) at Mar. 31, 2019		105,119,046
Beginning Balance at Dec. 31, 2018	$ 346,553	$ 10	720,025	(373,482)
Balance (in shares) at Dec. 31, 2018	104,337,508	104,337,508
Increase (Decrease) in Stockholders' Equity
Net loss	$ (46,077)
Ending Balance at Sep. 30, 2019	$ 329,978	$ 10	753,217	(423,249)
Balance (in shares) at Sep. 30, 2019	106,492,395	106,492,395
Beginning Balance at Mar. 31, 2019	$ 339,867	$ 10	731,394	(391,537)
Balance (in shares) at Mar. 31, 2019		105,119,046
Increase (Decrease) in Stockholders' Equity
Net loss	(24,060)			(24,060)
Stock-based compensation	16,061		16,061
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	(469)		(469)
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		776,563
Ending Balance at Jun. 30, 2019	331,399	$ 10	746,986	(415,597)
Balance (in shares) at Jun. 30, 2019		105,895,609
Increase (Decrease) in Stockholders' Equity
Net loss	(7,652)			(7,652)
Stock-based compensation	7,622		7,622
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	(1,391)		(1,391)
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		596,786
Ending Balance at Sep. 30, 2019	$ 329,978	$ 10	$ 753,217	$ (423,249)
Balance (in shares) at Sep. 30, 2019	106,492,395	106,492,395