Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Cumulative-effect adjustment to equity due to adoption of ASU 2016-02	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Accumulated Deficit Cumulative-effect adjustment to equity due to adoption of ASU 2016-02
Beginning Balance (in shares) at Dec. 31, 2018			90.8
Beginning Balance at Dec. 31, 2018	$ 480.8	$ 8.0	$ 0.1	$ 1,660.4	$ (2.0)	$ (1,177.7)	$ 8.0
Net income	3.0					3.0
Unrealized gain (loss) on debt securities available-for-sale, net of tax	3.6				3.6
Share-based compensation expense	54.0			54.0
Issuance of common stock for vested restricted stock units (in shares)			0.4
Issuance of common stock for stock option exercises (in shares)			0.8
Issuance of common stock for stock options	20.0			20.0
Issuance of common stock for employee stock purchase plan (in shares)			0.1
Issuance of common stock for employee stock purchase plan	5.1			5.1
Ending Balance (in shares) at Sep. 30, 2019			92.1
Ending Balance at Sep. 30, 2019	574.5		$ 0.1	1,739.5	1.6	(1,166.7)
Beginning Balance (in shares) at Jun. 30, 2019			91.5
Beginning Balance at Jun. 30, 2019	483.7		$ 0.1	1,703.5	0.6	(1,220.5)
Net income	53.8					53.8
Unrealized gain (loss) on debt securities available-for-sale, net of tax	1.0				1.0
Share-based compensation expense	20.3			20.3
Issuance of common stock for stock option exercises (in shares)			0.5
Issuance of common stock for stock options	13.1			13.1
Issuance of common stock for employee stock purchase plan (in shares)			0.1
Issuance of common stock for employee stock purchase plan	2.6			2.6
Ending Balance (in shares) at Sep. 30, 2019			92.1
Ending Balance at Sep. 30, 2019	574.5		$ 0.1	1,739.5	1.6	(1,166.7)
Beginning Balance (in shares) at Dec. 31, 2019			92.3
Beginning Balance at Dec. 31, 2019	636.9		$ 0.1	1,768.1	1.4	(1,132.7)
Net income	59.4					59.4
Unrealized gain (loss) on debt securities available-for-sale, net of tax	1.8				1.8
Share-based compensation expense	79.0			79.0
Issuance of common stock for vested restricted stock units (in shares)			0.5
Issuance of common stock for stock option exercises (in shares)			0.5
Issuance of common stock for stock options	21.6			21.6
Issuance of common stock for employee stock purchase plan (in shares)			0.1
Issuance of common stock for employee stock purchase plan	5.6			5.6
Ending Balance (in shares) at Sep. 30, 2020			93.4
Ending Balance at Sep. 30, 2020	804.3		$ 0.1	1,874.3	3.2	(1,073.3)
Beginning Balance (in shares) at Jun. 30, 2020			93.2
Beginning Balance at Jun. 30, 2020	831.2		$ 0.1	1,842.2	4.6	(1,015.7)
Net income	(57.6)					(57.6)
Unrealized gain (loss) on debt securities available-for-sale, net of tax	(1.4)				(1.4)
Share-based compensation expense	26.7			26.7
Issuance of common stock for stock option exercises (in shares)			0.1
Issuance of common stock for stock options	2.5			2.5
Issuance of common stock for employee stock purchase plan (in shares)			0.1
Issuance of common stock for employee stock purchase plan	2.9			2.9
Ending Balance (in shares) at Sep. 30, 2020			93.4
Ending Balance at Sep. 30, 2020	$ 804.3		$ 0.1	$ 1,874.3	$ 3.2	$ (1,073.3)