CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS’ EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Convertible Senior Notes, 1.375%	Cumulative Effect, Period of Adoption, Adjustment	[1]	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment	[1]	Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2019					62,685,000
Beginning balance at Dec. 31, 2019	$ 75.9		$ (1.1)		$ 0.1	$ 749.0	$ (672.0)	$ (1.1)		$ (1.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)					2,370,000
Issuance of common stock	477.5					477.5
Exercise of options to purchase common stock (in shares)					304,000
Exercise of options to purchase common stock	11.3					11.3
Issuance of shares for employee stock purchase plan (in shares)					19,000
Issuance of shares for employee stock purchase plan	2.9					2.9
Stock-based compensation expense	13.7					13.7
Restricted stock units vested, net of shares withheld for taxes (in shares)					226,000
Restricted stock units vested, net of shares withheld for taxes	(26.8)					(26.8)
Net (loss) income	12.3						12.3
Other comprehensive income (loss)	(2.4)									(2.4)
Ending balance (in shares) at Jun. 30, 2020					65,604,000
Ending balance at Jun. 30, 2020	563.3				$ 0.1	1,227.6	(660.8)			(3.6)
Beginning balance (in shares) at Mar. 31, 2020					63,058,000
Beginning balance at Mar. 31, 2020	59.0				$ 0.1	737.9	(675.2)			(3.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)					2,370,000
Issuance of common stock	477.5					477.5
Exercise of options to purchase common stock (in shares)					131,000
Exercise of options to purchase common stock	5.1					5.1
Issuance of shares for employee stock purchase plan (in shares)					19,000
Issuance of shares for employee stock purchase plan	2.9					2.9
Stock-based compensation expense	5.8					5.8
Restricted stock units vested, net of shares withheld for taxes (in shares)					26,000
Restricted stock units vested, net of shares withheld for taxes	(1.6)					(1.6)
Net (loss) income	14.4						14.4
Other comprehensive income (loss)	0.2									0.2
Ending balance (in shares) at Jun. 30, 2020					65,604,000
Ending balance at Jun. 30, 2020	$ 563.3				$ 0.1	1,227.6	(660.8)			(3.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Debt, interest rate		1.375%
Beginning balance (in shares) at Dec. 31, 2020	66,017,444				66,017,000
Beginning balance at Dec. 31, 2020	$ 603.6				$ 0.1	1,264.3	(666.3)			5.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)					164,000
Exercise of options to purchase common stock	6.2					6.2
Issuance of shares for employee stock purchase plan (in shares)					17,000
Issuance of shares for employee stock purchase plan	3.8					3.8
Stock-based compensation expense	17.6					17.6
Restricted stock units vested, net of shares withheld for taxes (in shares)					170,000
Restricted stock units vested, net of shares withheld for taxes	(27.3)					(27.3)
Extinguishment of conversion feature, net of issuance costs	(737.7)					(737.7)
Issuance of shares for debt repayment (in shares)					2,242,000
Issuance of shares for debt repayment	622.7					622.7
Net (loss) income	(25.0)						(25.0)
Other comprehensive income (loss)	$ (4.8)									(4.8)
Ending balance (in shares) at Jun. 30, 2021	68,610,078				68,610,000
Ending balance at Jun. 30, 2021	$ 459.1				$ 0.1	1,149.6	(691.3)			0.7
Beginning balance (in shares) at Mar. 31, 2021					66,213,000
Beginning balance at Mar. 31, 2021	585.3				$ 0.1	1,248.3	(666.3)			3.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)					121,000
Exercise of options to purchase common stock	4.7					4.7
Issuance of shares for employee stock purchase plan (in shares)					17,000
Issuance of shares for employee stock purchase plan	3.8					3.8
Stock-based compensation expense	9.0					9.0
Restricted stock units vested, net of shares withheld for taxes (in shares)					17,000
Restricted stock units vested, net of shares withheld for taxes	(1.2)					(1.2)
Extinguishment of conversion feature, net of issuance costs	(737.7)					(737.7)
Issuance of shares for debt repayment (in shares)					2,242,000
Issuance of shares for debt repayment	622.7					622.7				0.0
Net (loss) income	(25.0)						(25.0)
Other comprehensive income (loss)	$ (2.5)									(2.5)
Ending balance (in shares) at Jun. 30, 2021	68,610,078				68,610,000
Ending balance at Jun. 30, 2021	$ 459.1				$ 0.1	$ 1,149.6	$ (691.3)			$ 0.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Debt, interest rate		1.375%

[1] The Company recorded a cumulative effect adjustment to retained earnings to reflect the adoption of Accounting Standards Update 2016-13, Credit Losses (Topic 326). Refer to Note 2 of Item 8 of our Annual Report on Form 10-K for the year ended December 31, 2020.