Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Additional Paid in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Begining balance at Dec. 31, 2018	$ 480.8	$ 0.1	$ 1,660.4	$ (1.9)	$ (1,177.8)
Begining balance (in shares) at Dec. 31, 2018		90.8
Net income (loss)	(102.1)				(102.1)
Unrealized gain (loss) on debt securities available-for-sale, net of tax	1.7			1.7
Share-based compensation expense	15.8		15.8
Cumulative-effect adjustment to equity due to adoptionof ASU 2016-02 | ASU 2016-02	8.0				8.0
Issuance of common stock for vested restricted stock units (in shares)		0.4
Issuance of common stock for stock option exercises	2.6		2.6
Issuance of common stock for stock option exercises (in shares)		0.1
Issuance of common stock for employee stock purchase plan	2.5		2.5
Ending balance at Mar. 31, 2019	409.3	$ 0.1	1,681.3	(0.2)	(1,271.9)
Ending balance (in shares) at Mar. 31, 2019		91.3
Begining balance at Dec. 31, 2019	636.9	$ 0.1	1,768.1	1.4	(1,132.7)
Begining balance (in shares) at Dec. 31, 2019		92.3
Net income (loss)	37.4				37.4
Unrealized gain (loss) on debt securities available-for-sale, net of tax	(2.8)			(2.8)
Share-based compensation expense	22.8		22.8
Issuance of common stock for vested restricted stock units (in shares)		0.4
Issuance of common stock for stock option exercises	3.3		3.3
Issuance of common stock for stock option exercises (in shares)		0.1
Issuance of common stock for employee stock purchase plan	2.7		2.7
Ending balance at Mar. 31, 2020	$ 700.3	$ 0.1	$ 1,796.9	$ (1.4)	$ (1,095.3)
Ending balance (in shares) at Mar. 31, 2020		92.8