Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Stockholders' Equity Beginning Balance at Dec. 31, 2018	$ 522,801	$ 42	$ 1,218,710	$ 3	$ (695,954)
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2018		42,078
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(35,202)				(35,202)
Other comprehensive income (loss)	370			370
Issuance of common stock under employee equity plans	5,293		5,293
Issuance of common stock under employee equity plans (in shares)		380
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(606)		(606)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (in shares)		(12)
Stock-based compensation	4,223		4,223
Stockholders' Equity Ending Balance at Mar. 31, 2019	496,879	$ 42	1,227,620	373	(731,156)
Stockholders' Equity - Ending Balance (in shares) at Mar. 31, 2019		42,446
Stockholders' Equity Beginning Balance at Dec. 31, 2018	522,801	$ 42	1,218,710	3	(695,954)
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2018		42,078
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(72,965)
Stockholders' Equity Ending Balance at Jun. 30, 2019	465,715	$ 42	1,233,866	726	(768,919)
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2019		42,498
Stockholders' Equity Beginning Balance at Mar. 31, 2019	496,879	$ 42	1,227,620	373	(731,156)
Stockholders' Equity - Beginning Balance (in shares) at Mar. 31, 2019		42,446
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(37,763)				(37,763)
Other comprehensive income (loss)	353			353
Issuance of common stock under employee equity plans	888		888
Issuance of common stock under employee equity plans (in shares)		52
Stock-based compensation	5,358		5,358
Stockholders' Equity Ending Balance at Jun. 30, 2019	465,715	$ 42	1,233,866	726	(768,919)
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2019		42,498
Stockholders' Equity Beginning Balance at Dec. 31, 2019	245,059	$ 45	1,360,092	379	(1,115,457)
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2019		45,272
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(25,800)				(25,800)
Other comprehensive income (loss)	(167)			(167)
Issuance of common stock, net of offering costs	221,708	$ 10	221,698
Issuance of common stock, net of offering costs (in shares)		9,798
Issuance of common stock under employee equity plans	3,882		3,882
Issuance of common stock under employee equity plans (in shares)		297
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(569)		(569)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (in shares)		(26)
Stock-based compensation	6,394		6,394
Stockholders' Equity Ending Balance at Mar. 31, 2020	450,507	$ 55	1,591,497	212	(1,141,257)
Stockholders' Equity - Ending Balance (in shares) at Mar. 31, 2020		55,341
Stockholders' Equity Beginning Balance at Dec. 31, 2019	245,059	$ 45	1,360,092	379	(1,115,457)
Stockholders' Equity - Beginning Balance (in shares) at Dec. 31, 2019		45,272
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(79,124)
Stockholders' Equity Ending Balance at Jun. 30, 2020	405,143	$ 55	1,599,047	622	(1,194,581)
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2020		55,445
Stockholders' Equity Beginning Balance at Mar. 31, 2020	450,507	$ 55	1,591,497	212	(1,141,257)
Stockholders' Equity - Beginning Balance (in shares) at Mar. 31, 2020		55,341
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(53,324)				(53,324)
Other comprehensive income (loss)	410			410
Issuance of common stock under employee equity plans	616		616
Issuance of common stock under employee equity plans (in shares)		153
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(1,369)		(1,369)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (in shares)		(49)
Stock-based compensation	8,303		8,303
Stockholders' Equity Ending Balance at Jun. 30, 2020	$ 405,143	$ 55	$ 1,599,047	$ 622	$ (1,194,581)
Stockholders' Equity - Ending Balance (in shares) at Jun. 30, 2020		55,445