CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	2% Convertible Senior Notes	0.375% Convertible Senior Notes	1.25% Convertible Senior Notes	Common Stock	Additional Paid-in Capital	Additional Paid-in Capital 2% Convertible Senior Notes	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 (Income)
Beginning balance (in shares) at Dec. 31, 2017						58,319,000
Beginning balance at Dec. 31, 2017	$ 158.5	$ 20.4				$ 0.1	$ 866.2				$ (707.3)	$ 20.4	$ (0.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)						410,000
Exercise of options to purchase common stock	12.8						12.8
Issuance for employee stock purchase plan (in shares)						46,000
Issuance of shares for employee stock purchase plan	3.0						3.0
Stock-based compensation	37.5						37.5
Restricted stock units vested, net of shares withheld for taxes (in shares)						414,000
Restricted stock units vested, net of shares withheld for taxes	(17.8)						(17.8)
Extinguishment of conversion feature, net of issuance costs			$ (3.2)					$ (3.2)
Net income	3.3										3.3
Other comprehensive income (loss)	(2.4)												(2.4)
Ending balance (in shares) at Dec. 31, 2018						59,189,000
Ending 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)						1,340,000
Exercise of options to purchase common stock	46.6						46.6
Issuance for employee stock purchase plan (in shares)						51,000
Issuance of shares for employee stock purchase plan	4.3						4.3
Stock-based compensation	28.7						28.7
Restricted stock units vested, net of shares withheld for taxes (in shares)						230,000
Restricted stock units vested, net of shares withheld for taxes	$ (8.6)						(8.6)
Extinguishment of conversion feature, net of issuance costs				$ 207.8	$ (642.3)				$ 207.8	$ (642.3)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Issuance of shares for debt repayment (in shares)						1,875,000
Issuance of shares for debt repayment	$ 299.4						299.4
Purchase of capped call options	(85.4)						(85.4)
Net income	11.6										11.6
Other comprehensive income (loss)	$ 1.7												1.7
Ending balance (in shares) at Dec. 31, 2019	62,685,000					62,685,000
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]
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)	674,542					674,000
Exercise of options to purchase common stock	$ 25.7						25.7
Issuance for employee stock purchase plan (in shares)						38,000
Issuance of shares for employee stock purchase plan	6.0						6.0
Stock-based compensation	35.9						35.9
Restricted stock units vested, net of shares withheld for taxes (in shares)						250,000
Restricted stock units vested, net of shares withheld for taxes	(29.8)						(29.8)
Net income	6.8										6.8
Other comprehensive income (loss)	$ 6.7												6.7
Ending balance (in shares) at Dec. 31, 2020	66,017,000					66,017,000
Ending balance at Dec. 31, 2020	$ 603.6					$ 0.1	$ 1,264.3				$ (666.3)		$ 5.5