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	$ 1,025.5	$ 0.0	$ 0.2	$ 1,068.3	$ (0.8)	$ (0.1)	$ (42.2)	$ 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)			3.1
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(36.1)			(36.1)
Repurchase and retirement of common stock (in shares)			(1.6)
Repurchase and retirement of common stock	(117.4)			(10.0)			(107.4)
Stock-based compensation expense	130.4			130.4
Net unrealized gain (loss) on investments - net of tax	2.0				2.0
Net income	213.5						213.5
Balance, shares at Sep. 30, 2019			171.3
Balance at Sep. 30, 2019	1,217.9		$ 0.2	1,152.6	1.1		64.0
Balance, shares at Jun. 30, 2019			171.0
Balance at Jun. 30, 2019	1,139.4		$ 0.2	1,130.2	0.9		8.1
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	(17.8)			(17.8)
Repurchase and retirement of common stock (in shares)			(0.3)
Repurchase and retirement of common stock	(26.5)			(2.2)			(24.3)
Stock-based compensation expense	42.4			42.4
Net unrealized gain (loss) on investments - net of tax	0.2				0.2
Net income	80.2						80.2
Balance, shares at Sep. 30, 2019			171.3
Balance at Sep. 30, 2019	1,217.9		$ 0.2	1,152.6	1.1		64.0
Balance, shares at Dec. 31, 2019			171.7
Balance at Dec. 31, 2019	1,342.4		$ 0.2	1,180.3	1.1		160.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)			2.0
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(68.3)			(68.3)
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	143.0			143.0
Net unrealized gain (loss) on investments - net of tax	0.3				0.3
Net income	341.8						341.8
Balance, shares at Sep. 30, 2020			162.3
Balance at Sep. 30, 2020	713.2		$ 0.2	1,178.5	1.4		(466.9)
Balance, shares at Jun. 30, 2020			161.8
Balance at Jun. 30, 2020	566.7		$ 0.2	1,154.3	2.5		(590.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with equity incentive plans - net of tax withholding (in shares)			0.5
Issuance of common stock in connection with equity incentive plans - net of tax withholding	(25.1)			(25.1)
Stock-based compensation expense	49.3			49.3
Net unrealized gain (loss) on investments - net of tax	(1.1)				(1.1)
Net income	123.4						123.4
Balance, shares at Sep. 30, 2020			162.3
Balance at Sep. 30, 2020	$ 713.2		$ 0.2	$ 1,178.5	$ 1.4		$ (466.9)