CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018		81,046
Stockholders' equity, beginning balance at Dec. 31, 2018	$ 317,609	$ 8	$ 551,078	$ 2,226	$ (235,703)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings (in shares)		782
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings	732		732
Share-based compensation	19,616		19,616
Changes in other comprehensive loss	(375)			(375)
Net loss	(6,358)				(6,358)
Stockholders' equity, ending balance (in shares) at Mar. 31, 2019		81,828
Stockholders' equity, ending balance at Mar. 31, 2019	331,224	$ 8	571,426	1,851	(242,061)
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2018		81,046
Stockholders' equity, beginning balance at Dec. 31, 2018	317,609	$ 8	551,078	2,226	(235,703)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(28,350)
Stockholders' equity, ending balance (in shares) at Sep. 30, 2019		83,776
Stockholders' equity, ending balance at Sep. 30, 2019	369,494	$ 8	633,188	351	(264,053)
Stockholders' equity, beginning balance (in shares) at Mar. 31, 2019		81,828
Stockholders' equity, beginning balance at Mar. 31, 2019	331,224	$ 8	571,426	1,851	(242,061)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings (in shares)		1,046
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings	7,730		7,730
Share-based compensation	25,614		25,614
Changes in other comprehensive loss	242			242
Net loss	(9,243)				(9,243)
Stockholders' equity, ending balance (in shares) at Jun. 30, 2019		82,874
Stockholders' equity, ending balance at Jun. 30, 2019	355,567	$ 8	604,770	2,093	(251,304)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings (in shares)		902
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings	(1,116)		(1,116)
Share-based compensation	29,534		29,534
Changes in other comprehensive loss	(1,742)			(1,742)
Net loss	(12,749)				(12,749)
Stockholders' equity, ending balance (in shares) at Sep. 30, 2019		83,776
Stockholders' equity, ending balance at Sep. 30, 2019	369,494	$ 8	633,188	351	(264,053)
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2019		86,940
Stockholders' equity, beginning balance at Dec. 31, 2019	745,700	$ 9	1,033,053	1,948	(289,310)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings (in shares)		875
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings	(5,549)		(5,549)
Share-based compensation	37,001		37,001
Equity component from partial repurchase of 2023 convertible senior notes	(355,932)		(355,932)
Purchase of capped calls related to convertible senior notes	(60,900)		(60,900)
Equity component of convertible senior notes, net of issuance costs	192,442		192,442
Changes in other comprehensive loss	(1,617)			(1,617)
Net loss	(60,721)				(60,721)
Stockholders' equity, ending balance (in shares) at Mar. 31, 2020		87,815
Stockholders' equity, ending balance at Mar. 31, 2020	490,424	$ 9	840,115	331	(350,031)
Stockholders' equity, beginning balance (in shares) at Dec. 31, 2019		86,940
Stockholders' equity, beginning balance at Dec. 31, 2019	745,700	$ 9	1,033,053	1,948	(289,310)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(81,169)
Stockholders' equity, ending balance (in shares) at Sep. 30, 2020		89,585
Stockholders' equity, ending balance at Sep. 30, 2020	297,741	$ 9	664,689	3,522	(370,479)
Stockholders' equity, beginning balance (in shares) at Mar. 31, 2020		87,815
Stockholders' equity, beginning balance at Mar. 31, 2020	490,424	$ 9	840,115	331	(350,031)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings (in shares)		870
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings	8,550		8,550
Share-based compensation	52,129		52,129
Equity component from partial repurchase of 2023 convertible senior notes	(4,051)		(4,051)
Temporary equity reclassification	(6,756)		(6,756)
Changes in other comprehensive loss	771			771
Net loss	509				509
Stockholders' equity, ending balance (in shares) at Jun. 30, 2020		88,685
Stockholders' equity, ending balance at Jun. 30, 2020	541,576	$ 9	889,987	1,102	(349,522)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings (in shares)		580
Issuance of common stock in connection with Equity Incentive and Employee Stock Purchase plans, net of tax withholdings	(6,576)		(6,576)
Share-based compensation	53,640		53,640
Equity component of convertible senior notes, net of issuance costs (in shares)		320
Equity component from partial repurchase of 2023 convertible senior notes	(372,227)		(372,227)
Purchase of capped calls related to convertible senior notes	(41,795)		(41,795)
Equity component of convertible senior notes, net of issuance costs	136,838		136,838
Temporary equity reclassification	4,822		4,822
Changes in other comprehensive loss	2,420			2,420
Net loss	(20,957)				(20,957)
Stockholders' equity, ending balance (in shares) at Sep. 30, 2020		89,585
Stockholders' equity, ending balance at Sep. 30, 2020	$ 297,741	$ 9	$ 664,689	$ 3,522	$ (370,479)