CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Treasury Stock	Retained Earnings	Total
Balance at Dec. 31, 2017	$ 0.7	$ 1,854.3	$ (19.6)	$ (2,579.2)	$ 2,845.6	$ 2,101.8
Balance (in shares) at Dec. 31, 2017	69.9
Increase (Decrease) in Stockholders' Equity
Net income (loss)					523.9	523.9
Unrealized gain (loss) on available-for-sale securities			(3.6)			(3.6)
Defined benefit pension plan			1.0			1.0
Shares issued under employee stock purchase plan		3.9				3.9
Exercise of stock options		15.5				15.5
Exercise of stock options (in shares)	0.3
Share-based compensation		50.3				50.3
Balance at Sep. 30, 2018	$ 0.7	1,924.0	(22.2)	(2,579.2)	3,369.5	2,692.8
Balance (in shares) at Sep. 30, 2018	70.2
Balance at Jun. 30, 2018	$ 0.7	1,903.0	(21.9)	(2,579.2)	3,263.0	2,565.6
Balance (in shares) at Jun. 30, 2018	70.2
Increase (Decrease) in Stockholders' Equity
Net income (loss)					106.5	106.5
Unrealized gain (loss) on available-for-sale securities			(0.7)			(0.7)
Defined benefit pension plan			0.4			0.4
Shares issued under employee stock purchase plan		1.8				1.8
Exercise of stock options		0.6				0.6
Share-based compensation		18.6				18.6
Balance at Sep. 30, 2018	$ 0.7	1,924.0	(22.2)	(2,579.2)	3,369.5	2,692.8
Balance (in shares) at Sep. 30, 2018	70.2
Balance at Dec. 31, 2018	$ 0.7	1,940.2	(7.9)	(2,579.2)	3,434.8	2,788.6
Balance (in shares) at Dec. 31, 2018	70.2
Increase (Decrease) in Stockholders' Equity
Net income (loss)					(157.1)	(157.1)
Unrealized gain (loss) on available-for-sale securities			6.0			6.0
Defined benefit pension plan			(2.2)			(2.2)
Shares issued under employee stock purchase plan		4.1				4.1
Restricted stock units withheld for taxes		(2.1)				(2.1)
Common stock issued upon RSU vesting (in shares)	0.1
Exercise of stock options		9.9				9.9
Exercise of stock options (in shares)	0.2
Share-based compensation		63.0				63.0
Cumulative effect of accounting change					(5.1)	(5.1)
Reclassification from temporary equity to permanent equity		10.8				10.8
Deconsolidation of variable interest entity		(0.1)				(0.1)
Balance at Sep. 30, 2019	$ 0.7	2,025.8	(4.1)	(2,579.2)	3,272.6	2,715.8
Balance (in shares) at Sep. 30, 2019	70.5
Balance at Jun. 30, 2019	$ 0.7	2,001.7	(5.0)	(2,579.2)	3,140.2	2,558.4
Balance (in shares) at Jun. 30, 2019	70.5
Increase (Decrease) in Stockholders' Equity
Net income (loss)					132.4	132.4
Unrealized gain (loss) on available-for-sale securities			1.0			1.0
Defined benefit pension plan			(0.1)			(0.1)
Shares issued under employee stock purchase plan		1.9				1.9
Restricted stock units withheld for taxes		(0.2)				(0.2)
Share-based compensation		22.5				22.5
Deconsolidation of variable interest entity		(0.1)				(0.1)
Balance at Sep. 30, 2019	$ 0.7	$ 2,025.8	$ (4.1)	$ (2,579.2)	$ 3,272.6	$ 2,715.8
Balance (in shares) at Sep. 30, 2019	70.5