Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Balance, shares at Dec. 31, 2018		169.8
Balance at Dec. 31, 2018	$ 1,010.2	$ 0.2	$ 1,068.3	$ (0.8)	$ (57.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)		1.7
Issuance of common stock in connection with equity incentive plans - net of tax withholding	1.6		1.6
Repurchase and retirement of common stock (in shares)		(0.8)
Repurchase and retirement of common stock	(56.2)		(4.8)		(51.4)
Stock-based compensation expense	43.0		43.0
Net unrealized gain (loss) on investments - net of tax	0.7			0.7
Net income	58.8				58.8
Balance, shares at Mar. 31, 2019		170.7
Balance at Mar. 31, 2019	1,058.1	$ 0.2	1,108.1	(0.2)	(50.0)
Balance, shares at Dec. 31, 2019		171.7
Balance at Dec. 31, 2019	1,321.9	$ 0.2	1,180.3	1.1	140.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)		0.9
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(30.3)		(30.3)
Repurchase and retirement of common stock (in shares)		(10.0)
Repurchase and retirement of common stock	(899.9)		(66.4)		(833.5)
Stock-based compensation expense	44.8		44.8
Net unrealized gain (loss) on investments - net of tax	(4.2)			(4.2)
Net income	104.0
Balance, shares at Mar. 31, 2020		162.6
Balance at Mar. 31, 2020	$ 536.3	$ 0.2	$ 1,128.4	$ (3.1)	$ (589.2)