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, 2018	$ 53	$ 2,034,142	$ (44,634)	$ (1,843,092)	$ 146,469
Beginning Balance (in shares) at Dec. 31, 2018	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
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				(86,421)	(86,421)
Ending Balance at Mar. 31, 2019	$ 53	2,048,874	(44,564)	(1,929,513)	74,850
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
Net unrealized gain (loss) on available-for-sale securities					152
Foreign currency translation adjustments					(6)
Net loss					(206,850)
Ending Balance at Jun. 30, 2019	$ 53	2,064,393	(44,488)	(2,049,940)	(29,982)
Ending Balance (in shares) at Jun. 30, 2019	53,170,479
Beginning Balance at Mar. 31, 2019	$ 53	2,048,874	(44,564)	(1,929,513)	74,850
Beginning Balance (in shares) at Mar. 31, 2019	52,994,050
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 under employee stock purchase plan		1,166			1,166
Issuance of common stock under employee stock purchase plan (in shares)	92,275
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				(120,427)	(120,427)
Ending Balance at Jun. 30, 2019	$ 53	2,064,393	(44,488)	(2,049,940)	(29,982)
Ending Balance (in shares) at Jun. 30, 2019	53,170,479
Beginning Balance at Dec. 31, 2019	$ 55	2,114,068	(44,865)	(2,243,515)	$ (174,257)
Beginning Balance (in shares) at Dec. 31, 2019	54,956,341				54,956,341
Exercise of stock options		2			$ 2
Exercise of stock options (in shares)	759
Issuance of common stock from vesting of restricted stock units (in shares)	662,323
Share-based compensation expense		12,961			12,961
Net unrealized gain (loss) on available-for-sale securities			78		78
Foreign currency translation adjustments			(103)		(103)
Convertible senior notes conversion	$ 18	133,640			133,658
Convertible senior notes conversion (in shares)	17,877,164
Net loss				(99,332)	(99,332)
Ending Balance at Mar. 31, 2020	$ 73	2,260,671	(44,890)	(2,342,847)	(126,993)
Ending Balance (in shares) at Mar. 31, 2020	73,496,587
Beginning Balance at Dec. 31, 2019	$ 55	2,114,068	(44,865)	(2,243,515)	$ (174,257)
Beginning Balance (in shares) at Dec. 31, 2019	54,956,341				54,956,341
Exercise of stock options (in shares)					24,153
Net unrealized gain (loss) on available-for-sale securities					$ (6)
Foreign currency translation adjustments					(42)
Net loss					(191,579)
Ending Balance at Jun. 30, 2020	$ 88	2,382,544	(44,913)	(2,435,094)	$ (97,375)
Ending Balance (in shares) at Jun. 30, 2020	88,196,705				88,196,705
Beginning Balance at Mar. 31, 2020	$ 73	2,260,671	(44,890)	(2,342,847)	$ (126,993)
Beginning Balance (in shares) at Mar. 31, 2020	73,496,587
Exercise of stock options		(41)			(41)
Exercise of stock options (in shares)	6,661
Issuance of common stock from vesting of restricted stock units (in shares)	113,461
Issuance of common stock under employee stock purchase plan		907			907
Issuance of common stock under employee stock purchase plan (in shares)	158,126
Share-based compensation expense		13,313			13,313
Net unrealized gain (loss) on available-for-sale securities			(84)		(84)
Foreign currency translation adjustments			61		61
Convertible senior notes conversion	$ 4	24,278			24,282
Convertible senior notes conversion (in shares)	3,331,870
Net loss				(92,247)	(92,247)
Ending Balance at Jun. 30, 2020	$ 88	2,382,544	$ (44,913)	$ (2,435,094)	$ (97,375)
Ending Balance (in shares) at Jun. 30, 2020	88,196,705				88,196,705
Issuance of common stock, net of issuance costs	$ 11	$ 83,416			$ 83,427
Issuance of common stock, net of issuance costs (in shares)	11,090,000