CONDENSED CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Loss) Income	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2017		448,686,791	65,206,999
Balance at beginning of period at Dec. 31, 2017	$ (551,365)	$ 1,176,661	$ 6	$ 71,679	$ (1,157)	$ (621,893)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			335,053
Vesting of restricted common stock	0
Issuance of Series G redeemable convertible preferred stock, net of issuance costs of $10,517 (in shares)		55,666,004
Issuance of Series G redeemable convertible preferred stock, net of issuance costs of $10,517	152	$ 549,413		152
Exercise of options to purchase common stock, net (in shares)			1,789
Exercise of options to purchase common stock, net	22			22
Stock-based compensation	11,851			11,851
Unrealized gain (loss) on marketable securities	(1,993)				(1,993)
Net loss	(72,376)					(72,376)
Balance at end of period (in shares) at Mar. 31, 2018		504,352,795	65,543,841
Balance at end of period at Mar. 31, 2018	(613,709)	$ 1,726,074	$ 6	83,704	(3,150)	(694,269)
Balance at beginning of period (in shares) at Dec. 31, 2018		0	328,798,904
Balance at beginning of period at Dec. 31, 2018	1,530,241	$ 0	$ 33	2,538,155	(1,320)	(1,006,627)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)			48,911
Vesting of restricted common stock	$ 0
Exercise of options to purchase common stock, net (in shares)	5,525		5,525
Exercise of options to purchase common stock, net	$ 57			57
Stock-based compensation	18,497			18,497
Unrealized gain (loss) on marketable securities	1,911				1,911
Net loss	(132,657)					(132,657)
Balance at end of period (in shares) at Mar. 31, 2019		0	328,853,340
Balance at end of period at Mar. 31, 2019	$ 1,446,033	$ 0	$ 33	$ 2,556,709	$ 591	$ (1,111,300)