Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Cumulative-effect adjustment from adoption of ASU 2018-02	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Cumulative-effect adjustment from adoption of ASU 2018-02	Retained Earnings (Accumulated Deficit) [Member]	Retained Earnings (Accumulated Deficit) [Member] Cumulative-effect adjustment from adoption of ASU 2018-02
Balance, shares at Dec. 31, 2018			169.8
Balance at Dec. 31, 2018	$ 1,010.2	$ 0.0	$ 0.2	$ 1,068.3	$ (0.8)	$ (0.1)	$ (57.5)	$ 0.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)			2.5
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(18.3)			(18.3)
Repurchase and retirement of common stock (in shares)			(1.3)
Repurchase and retirement of common stock	(90.9)			(7.8)			(83.1)
Stock-based compensation expense	88.0			88.0
Net unrealized gain on investments - net of tax	1.8				1.8
Net income	131.5
Balance, shares at Jun. 30, 2019			171.0
Balance at Jun. 30, 2019	1,122.3		$ 0.2	1,130.2	0.9		(9.0)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)			0.8
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(19.9)			(19.9)
Repurchase and retirement of common stock (in shares)			(0.5)
Repurchase and retirement of common stock	(34.7)			(3.0)			(31.7)
Stock-based compensation expense	45.0			45.0
Net unrealized gain on investments - net of tax	1.1				1.1
Net income	72.7						72.7
Balance, shares at Jun. 30, 2019			171.0
Balance at Jun. 30, 2019	1,122.3		$ 0.2	1,130.2	0.9		(9.0)
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)			1.5
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(43.2)			(43.2)
Repurchase and retirement of common stock (in shares)			(11.4)
Repurchase and retirement of common stock	(1,046.0)			(76.5)			(969.5)
Stock-based compensation expense	93.7			93.7
Net unrealized gain on investments - net of tax	1.4				1.4
Net income	216.1
Balance, shares at Jun. 30, 2020			161.8
Balance at Jun. 30, 2020	543.9		$ 0.2	1,154.3	2.5		(613.1)
Balance, shares at Mar. 31, 2020			162.6
Balance at Mar. 31, 2020	536.3		$ 0.2	1,128.4	(3.1)		(589.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)			0.6
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(12.9)			(12.9)
Repurchase and retirement of common stock (in shares)			(1.4)
Repurchase and retirement of common stock	(146.1)			(10.1)			(136.0)
Stock-based compensation expense	48.9			48.9
Net unrealized gain on investments - net of tax	5.6				5.6
Net income	112.1
Balance, shares at Jun. 30, 2020			161.8
Balance at Jun. 30, 2020	$ 543.9		$ 0.2	$ 1,154.3	$ 2.5		$ (613.1)