Consolidated Statements of Changes in Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	At-the-Market Offering	Common Shares	Common Shares At-the-Market Offering	Accumulated Deficit	Additional Paid-in Capital	Additional Paid-in Capital At-the-Market Offering	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2018			31.2
Beginning balance at Dec. 31, 2018	$ 136.6		$ 0.0		$ (302.3)	$ 439.1		$ (0.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	20.1					20.1
Issuance of common stock, net (in shares)			6.6
Issuances of common stock, net	137.1		$ 0.0			137.1
Net loss	(70.3)				(70.3)
Restricted stock vesting (in shares)			0.5
Restricted stock vesting	0.0		$ 0.0			0.0
Proceeds from exercise of stock options (in shares)			0.2
Proceeds from exercise of stock options	2.8		$ 0.0			2.8
Unrealized gain (loss) on available-for-sale securities	0.3							0.3
Ending balance (in shares) at Dec. 31, 2019			38.5
Ending balance at Dec. 31, 2019	226.6		$ 0.0		(372.6)	599.1		0.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	30.2					30.2
Issuance of common stock, net (in shares)			6.6	2.5
Issuances of common stock, net	431.9	$ 64.1	$ 0.0	$ 0.0		431.9	$ 64.1
Net loss	(119.3)				(119.3)
Restricted stock vesting (in shares)			0.4
Restricted stock vesting	0.0		$ 0.0			0.0
Proceeds from exercise of stock options (in shares)			0.5
Proceeds from exercise of stock options	8.2		$ 0.0			8.2
Unrealized gain (loss) on available-for-sale securities	$ 0.5							0.5
Ending balance (in shares) at Dec. 31, 2020	48.5		48.5
Ending balance at Dec. 31, 2020	$ 642.2		$ 0.0		(491.9)	1,133.5		0.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	57.1					57.1
Issuance of common stock, net (in shares)			3.5
Issuances of common stock, net	259.9		$ 0.0			259.9
Net loss	(191.0)				(191.0)
Restricted stock vesting (in shares)			0.2
Restricted stock vesting	0.0		$ 0.0			0.0
Proceeds from exercise of stock options (in shares)			0.8
Proceeds from exercise of stock options	18.7		$ 0.0			18.7
Unrealized gain (loss) on available-for-sale securities	$ (5.2)							(5.2)
Ending balance (in shares) at Dec. 31, 2021	53.0		53.0
Ending balance at Dec. 31, 2021	$ 781.7		$ 0.0		$ (682.9)	$ 1,469.2		$ (4.6)