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, 2017	$ 301,521	$ 35	$ 873,526	$ 0	$ (572,040)
Stockholders' Equity - Beginning Balance (shares) at Dec. 31, 2017		34,808
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(30,180)				(30,180)
Issuance of common stock under employee equity plans	1,930		1,930
Issuance of common stock under employee equity plans (shares)		198
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(1,411)		(1,411)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (shares)		(33)
Stock-based compensation	1,912		1,912
Stockholders' Equity Ending Balance at Mar. 31, 2018	273,772	$ 35	875,957	0	(602,220)
Stockholders' Equity - Ending Balance (shares) at Mar. 31, 2018		34,973
Stockholders' Equity Beginning Balance at Dec. 31, 2017	301,521	$ 35	873,526	0	(572,040)
Stockholders' Equity - Beginning Balance (shares) at Dec. 31, 2017		34,808
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(101,481)
Stockholders' Equity Ending Balance at Sep. 30, 2018	538,780	$ 42	1,212,305	(46)	(673,521)
Stockholders' Equity - Ending Balance (shares) at Sep. 30, 2018		41,925
Stockholders' Equity Beginning Balance at Mar. 31, 2018	273,772	$ 35	875,957	0	(602,220)
Stockholders' Equity - Beginning Balance (shares) at Mar. 31, 2018		34,973
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(29,037)				(29,037)
Issuance of common stock, net of offering costs	30,252	$ 1	30,251
Issuance of common stock, net of offering costs (shares)		740
Issuance of common stock under employee equity plans	1,838		1,838
Issuance of common stock under employee equity plans (shares)		114
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(18)		(18)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (shares)		0
Stock-based compensation	3,059		3,059
Stockholders' Equity Ending Balance at Jun. 30, 2018	279,866	$ 36	911,087	0	(631,257)
Stockholders' Equity - Ending Balance (shares) at Jun. 30, 2018		35,827
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(42,264)				(42,264)
Other comprehensive income (loss)	(46)			(46)
Issuance of common stock, net of offering costs	292,886	$ 6	292,880
Issuance of common stock, net of offering costs (shares)		6,000
Issuance of common stock under employee equity plans	1,357		1,357
Issuance of common stock under employee equity plans (shares)		98
Stock-based compensation	6,981		6,981
Stockholders' Equity Ending Balance at Sep. 30, 2018	538,780	$ 42	1,212,305	(46)	(673,521)
Stockholders' Equity - Ending Balance (shares) at Sep. 30, 2018		41,925
Stockholders' Equity Beginning Balance at Dec. 31, 2018	522,801	$ 42	1,218,710	3	(695,954)
Stockholders' Equity - Beginning Balance (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 (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 (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 (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 (shares) at Dec. 31, 2018		42,078
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(363,443)
Stockholders' Equity Ending Balance at Sep. 30, 2019	250,592	$ 44	1,309,523	422	(1,059,397)
Stockholders' Equity - Ending Balance (shares) at Sep. 30, 2019		44,251
Stockholders' Equity Beginning Balance at Mar. 31, 2019	496,879	$ 42	1,227,620	373	(731,156)
Stockholders' Equity - Beginning Balance (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 (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 (shares) at Jun. 30, 2019		42,498
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(290,478)				(290,478)
Other comprehensive income (loss)	(304)			(304)
Issuance of common stock as consideration for asset acquisition	68,124	$ 2	68,122
Issuance of common stock as consideration for asset acquisition (shares)		1,595
Issuance of common stock under employee equity plans	2,238		2,238
Issuance of common stock under employee equity plans (shares)		163
Shares repurchased for tax withholdings related to net share settlement of employee equity awards	(138)		(138)
Shares repurchased for tax withholdings related to net share settlement of employee equity awards (shares)		(5)
Stock-based compensation	5,435		5,435
Stockholders' Equity Ending Balance at Sep. 30, 2019	$ 250,592	$ 44	$ 1,309,523	$ 422	$ (1,059,397)
Stockholders' Equity - Ending Balance (shares) at Sep. 30, 2019		44,251