Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjustment	Additional Paid-in Capital	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)			716,687
Exercise of options to purchase common stock	23.6				23.6
Issuance of employee stock purchase plan (in shares)			27,613
Issuance of employee stock purchase plan	2.0				2.0
Stock-based compensation	14.1				14.1
Restricted stock units vested, net of shares withheld for taxes (in shares)			216,868
Restricted stock units vested, net of shares withheld for taxes	(7.9)				(7.9)
Net income	5.8					5.8
Other comprehensive income	0.1							0.1
Ending balance (in shares) at Jun. 30, 2019			60,149,926
Ending balance at Jun. 30, 2019	249.8		$ 0.1		930.3	(677.8)		(2.8)
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 Mar. 31, 2019			59,638,439
Beginning balance at Mar. 31, 2019	$ 223.6		$ 0.1		905.8	(679.2)		(3.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of options to purchase common stock (in shares)			447,214
Exercise of options to purchase common stock	14.6				14.6
Issuance of employee stock purchase plan (in shares)			27,613
Issuance of employee stock purchase plan	2.0				2.0
Stock-based compensation	8.3				8.3
Restricted stock units vested, net of shares withheld for taxes (in shares)			36,660
Restricted stock units vested, net of shares withheld for taxes	(0.4)				(0.4)
Net income	1.4					1.4
Other comprehensive income	0.3							0.3
Ending balance (in shares) at Jun. 30, 2019			60,149,926
Ending balance at Jun. 30, 2019	$ 249.8		$ 0.1		930.3	(677.8)		(2.8)
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)			304,374
Exercise of options to purchase common stock	11.3				11.3
Issuance of employee stock purchase plan (in shares)			18,685
Issuance of employee stock purchase plan	2.9				2.9
Stock-based compensation	13.7				13.7
Restricted stock units vested, net of shares withheld for taxes (in shares)			226,128
Restricted stock units vested, net of shares withheld for taxes	(26.8)				(26.8)
Net income	12.3					12.3
Other comprehensive income	$ (2.4)							(2.4)
Ending balance (in shares) at Jun. 30, 2020	65,604,347		65,604,347
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			63,057,874
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)			2,369,668
Issuance of common stock	477.5				477.5
Exercise of options to purchase common stock (in shares)			131,542.0
Exercise of options to purchase common stock	5.1				5.1
Issuance of employee stock purchase plan (in shares)			18,685
Issuance of employee stock purchase plan	2.9				2.9
Stock-based compensation	5.8				5.8
Restricted stock units vested, net of shares withheld for taxes (in shares)			26,578
Restricted stock units vested, net of shares withheld for taxes	(1.6)				(1.6)
Net income	14.4					14.4
Other comprehensive income	$ 0.2							0.2
Ending balance (in shares) at Jun. 30, 2020	65,604,347		65,604,347
Ending balance at Jun. 30, 2020	$ 563.3		$ 0.1		$ 1,227.6	$ (660.8)		$ (3.6)