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, 2016	$ 0.7	$ 1,813.5	$ (16.8)	$ (2,379.6)	$ 2,433.5	$ 1,851.3
Balance (in shares) at Dec. 31, 2016	69.3
Increase (Decrease) in Stockholders' Equity
Net (loss) income					417.9	417.9
Foreign currency translation adjustments			0.2			0.2
Unrealized gain (loss) on available-for-sale securities			(1.9)			(1.9)
Defined benefit pension plan			(1.1)			(1.1)
Shares issued under employee stock purchase plan		4.1				4.1
Shares issued under employee stock purchase plan (in shares)	0.1
Shares issued upon expiration of warrants		(53.2)		53.2
Repurchase of shares		2.8		(252.8)		(250.0)
Exercise of stock options		39.9				39.9
Exercise of stock options (in shares)	0.5
Share-based compensation		46.4				46.4
Cumulative effect of accounting change		0.7			(5.8)	(5.1)
Consolidation of variable interest entity		0.1				0.1
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 (loss) income					589.2	589.2
Unrealized gain (loss) on available-for-sale securities			(0.4)			(0.4)
Defined benefit pension plan			12.1			12.1
Shares issued under employee stock purchase plan		3.9				3.9
Restricted stock units withheld for taxes		(0.1)				(0.1)
Exercise of stock options		15.6				15.6
Exercise of stock options (in shares)	0.3
Share-based compensation		66.5				66.5
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 (loss) income					(104.5)	(104.5)
Unrealized gain (loss) on available-for-sale securities			6.0			6.0
Defined benefit pension plan			(12.3)			(12.3)
Shares issued under employee stock purchase plan		4.1				4.1
Common stock issued upon RSUs vesting (in shares)	0.1
Restricted stock units withheld for taxes		(2.1)				(2.1)
Exercise of stock options		9.9				9.9
Exercise of stock options (in shares)	0.2
Share-based compensation		85.1				85.1
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 Dec. 31, 2019	$ 0.7	$ 2,047.9	$ (14.2)	$ (2,579.2)	$ 3,325.2	$ 2,780.4
Balance (in shares) at Dec. 31, 2019	70.5