CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Beginning Balance at Dec. 31, 2017	$ 51	$ 1,887,197	$ (42,173)	$ (1,477,441)	$ 367,634
Beginning Balance (in shares) at Dec. 31, 2017	50,565,119
Exercise of stock options		514			514
Exercise of stock options (in shares)	21,463
Issuance of common stock from vesting of restricted stock units (in shares)	110,889
Issuance of common stock from vesting of restricted stock units (in shares)	110,889
Share-based compensation expense		11,913			11,913
Net unrealized gain (loss) on available-for-sale securities			(5)		(5)
Foreign currency translation adjustments			1,517		1,517
Adoption of new revenue recognition standard				2,356	2,356
Net loss				(77,694)	(77,694)
Ending Balance at Mar. 31, 2018	$ 51	1,899,624	(40,661)	(1,552,779)	306,235
Ending Balance (in shares) at Mar. 31, 2018	50,697,471
Beginning Balance at Dec. 31, 2017	$ 51	1,887,197	(42,173)	(1,477,441)	367,634
Beginning Balance (in shares) at Dec. 31, 2017	50,565,119
Net unrealized gain (loss) on available-for-sale securities					71
Foreign currency translation adjustments					(2,448)
Net loss					(268,750)
Ending Balance at Sep. 30, 2018	$ 53	2,021,997	(44,550)	(1,743,834)	233,666
Ending Balance (in shares) at Sep. 30, 2018	52,708,767
Beginning Balance at Mar. 31, 2018	$ 51	1,899,624	(40,661)	(1,552,779)	306,235
Beginning Balance (in shares) at Mar. 31, 2018	50,697,471
Issuance of common stock, net of issuance costs	$ 2	93,752			93,754
Issuance of common stock, net of issuance costs (in shares)	1,837,898
Issuance of common stock under employee stock purchase plan		1,377			1,377
Issuance of common stock under employee stock purchase plan (in shares)	35,636
Exercise of stock options		703			703
Exercise of stock options (in shares)	25,753
Issuance of common stock from vesting of restricted stock units (in shares)	40,617
Issuance of common stock from vesting of restricted stock units (in shares)	40,617
Share-based compensation expense		14,855			14,855
Net unrealized gain (loss) on available-for-sale securities			86		86
Foreign currency translation adjustments			(3,470)		(3,470)
Net loss				(101,191)	(101,191)
Ending Balance at Jun. 30, 2018	$ 53	2,010,311	(44,045)	(1,653,970)	312,349
Ending Balance (in shares) at Jun. 30, 2018	52,637,375
Issuance of common stock, net of issuance costs		135			135
Exercise of stock options		604			604
Exercise of stock options (in shares)	23,514
Issuance of common stock from vesting of restricted stock units (in shares)	47,878
Issuance of common stock from vesting of restricted stock units (in shares)	47,878
Share-based compensation expense		10,947			10,947
Net unrealized gain (loss) on available-for-sale securities			(10)		(10)
Foreign currency translation adjustments			(495)		(495)
Net loss				(89,864)	(89,864)
Ending Balance at Sep. 30, 2018	$ 53	2,021,997	(44,550)	(1,743,834)	233,666
Ending Balance (in shares) at Sep. 30, 2018	52,708,767
Beginning Balance at Dec. 31, 2018	$ 53	2,034,142	(44,634)	(1,843,092)	$ 146,469
Beginning Balance (in shares) at Dec. 31, 2018	52,797,516				52,797,516
Exercise of stock options		1,093			$ 1,093
Exercise of stock options (in shares)	83,132
Issuance of common stock from vesting of restricted stock units (in shares)	113,402
Issuance of common stock from vesting of restricted stock units (in shares)	113,402
Share-based compensation expense		13,639			13,639
Net unrealized gain (loss) on available-for-sale securities			75		75
Foreign currency translation adjustments			(5)		(5)
Net loss				(86,420)	(86,420)
Ending Balance at Mar. 31, 2019	$ 53	2,048,874	(44,564)	(1,929,512)	74,851
Ending Balance (in shares) at Mar. 31, 2019	52,994,050
Beginning Balance at Dec. 31, 2018	$ 53	2,034,142	(44,634)	(1,843,092)	$ 146,469
Beginning Balance (in shares) at Dec. 31, 2018	52,797,516				52,797,516
Exercise of stock options (in shares)					188,355
Net unrealized gain (loss) on available-for-sale securities					$ 84
Foreign currency translation adjustments					(149)
Net loss					(300,923)
Ending Balance at Sep. 30, 2019	$ 55	2,101,162	(44,699)	(2,144,013)	$ (87,495)
Ending Balance (in shares) at Sep. 30, 2019	54,805,525				54,805,525
Beginning Balance at Mar. 31, 2019	$ 53	2,048,874	(44,564)	(1,929,512)	$ 74,851
Beginning Balance (in shares) at Mar. 31, 2019	52,994,050
Issuance of common stock under employee stock purchase plan		1,166			1,166
Issuance of common stock under employee stock purchase plan (in shares)	92,275
Exercise of stock options		223			223
Exercise of stock options (in shares)	20,741
Issuance of common stock from vesting of restricted stock units (in shares)	63,413
Issuance of common stock from vesting of restricted stock units (in shares)	63,413
Share-based compensation expense		14,130			14,130
Net unrealized gain (loss) on available-for-sale securities			77		77
Foreign currency translation adjustments			(1)		(1)
Net loss				(120,426)	(120,426)
Ending Balance at Jun. 30, 2019	$ 53	2,064,393	(44,488)	(2,049,938)	(29,980)
Ending Balance (in shares) at Jun. 30, 2019	53,170,479
Exercise of stock options		45			45
Exercise of stock options (in shares)	84,482
Issuance of common stock from vesting of restricted stock units (in shares)	68,506
Issuance of common stock from vesting of restricted stock units (in shares)	68,506
Share-based compensation expense		13,979			13,979
Litigation settlement	$ 2	22,745			22,747
Litigation settlement (in shares)	1,482,058
Net unrealized gain (loss) on available-for-sale securities			(68)		(68)
Foreign currency translation adjustments			(143)		(143)
Net loss				(94,075)	(94,075)
Ending Balance at Sep. 30, 2019	$ 55	$ 2,101,162	$ (44,699)	$ (2,144,013)	$ (87,495)
Ending Balance (in shares) at Sep. 30, 2019	54,805,525				54,805,525