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		13,640			13,640
Net unrealized gain 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	2,048,875	(44,564)	(1,929,513)	74,851
Ending Balance (in shares) at Mar. 31, 2019	52,994,050
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				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 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)	$ (2,342,848)	$ (126,994)
Ending Balance (in shares) at Mar. 31, 2020	73,496,587				73,496,587