CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS’ EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	0.375% Convertible Senior Notes	1.25% Convertible Senior Notes	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjustment	Additional Paid-in Capital	Additional Paid-in Capital 0.375% Convertible Senior Notes	Additional Paid-in Capital 1.25% Convertible Senior Notes	Accumulated Deficit	Accumulated Deficit Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss
Beginning balance (in shares) at Dec. 31, 2018					59,188,758
Beginning balance at Dec. 31, 2018	$ 212.1				$ 0.1		$ 898.5			$ (683.6)		$ (2.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)					1,188,043
Exercise of options to purchase common stock	40.9						40.9
Issuance of employee stock purchase plan (in shares)					27,613
Issuance of shares for employee stock purchase plan	2.0						2.0
Stock-based compensation	20.7						20.7
Restricted stock units vested, net of shares withheld for taxes (in shares)					225,063
Restricted stock units vested, net of shares withheld for taxes	(8.2)						(8.2)
Conversion feature, net of issuance costs			$ 207.8	$ (438.6)				$ 207.8	$ (438.6)
Issuance of shares for debt repayment (in shares)					1,157,492
Issuance of shares for debt repayment	194.4						194.4
Purchase of capped call options	(85.4)						(85.4)
Net income	6.6									6.6
Other comprehensive income (loss)	(1.7)											(1.7)
Ending balance (in shares) at Sep. 30, 2019					61,786,969
Ending balance at Sep. 30, 2019	150.6				$ 0.1		832.1			(677.0)		(4.6)
Beginning balance (in shares) at Dec. 31, 2018					59,188,758
Beginning balance at Dec. 31, 2018	$ 212.1				$ 0.1		898.5			(683.6)		(2.9)
Ending balance (in shares) at Dec. 31, 2019	62,685,492				62,685,492	0
Ending 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]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Beginning balance (in shares) at Jun. 30, 2019					60,149,926
Beginning balance at Jun. 30, 2019	$ 249.8				$ 0.1		930.3			(677.8)		(2.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)					471,356
Exercise of options to purchase common stock	17.3						17.3
Stock-based compensation	6.6						6.6
Restricted stock units vested, net of shares withheld for taxes (in shares)					8,195
Restricted stock units vested, net of shares withheld for taxes	(0.3)						(0.3)
Conversion feature, net of issuance costs			$ 207.8	$ (438.6)				$ 207.8	$ (438.6)
Issuance of shares for debt repayment (in shares)					1,157,492
Issuance of shares for debt repayment	194.4						194.4
Purchase of capped call options	(85.4)						(85.4)
Net income	0.8									0.8
Other comprehensive income (loss)	(1.8)											(1.8)
Ending balance (in shares) at Sep. 30, 2019					61,786,969
Ending balance at Sep. 30, 2019	$ 150.6				$ 0.1		832.1			(677.0)		(4.6)
Beginning balance (in shares) at Dec. 31, 2019	62,685,492				62,685,492	0
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)					2,369,668
Issuance of common stock	477.5						477.5
Exercise of options to purchase common stock (in shares)					568,830
Exercise of options to purchase common stock	21.9						21.9
Issuance of employee stock purchase plan (in shares)					18,685
Issuance of shares for employee stock purchase plan	2.9						2.9
Stock-based compensation	20.3						20.3
Restricted stock units vested, net of shares withheld for taxes (in shares)					229,061
Restricted stock units vested, net of shares withheld for taxes	(26.9)						(26.9)
Net income	23.9									23.9
Other comprehensive income (loss)	$ 0.6											0.6
Ending balance (in shares) at Sep. 30, 2020	65,871,736				65,871,736
Ending balance at Sep. 30, 2020	$ 595.0				$ 0.1		1,244.7			(649.2)		(0.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Beginning balance (in shares) at Jun. 30, 2020					65,604,347
Beginning balance at Jun. 30, 2020	$ 563.3				$ 0.1		1,227.6			(660.8)		(3.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)					264,456
Exercise of options to purchase common stock	10.6						10.6
Stock-based compensation	6.6						6.6
Restricted stock units vested, net of shares withheld for taxes (in shares)					2,933
Restricted stock units vested, net of shares withheld for taxes	(0.1)						(0.1)
Net income	11.6									11.6
Other comprehensive income (loss)	$ 3.0											3.0
Ending balance (in shares) at Sep. 30, 2020	65,871,736				65,871,736
Ending balance at Sep. 30, 2020	$ 595.0				$ 0.1		$ 1,244.7			$ (649.2)		$ (0.6)