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 (loss) income					244.5	244.5
Unrealized gain (loss) on available-for-sale securities			(2.4)			(2.4)
Defined benefit pension plan			0.3			0.3
Shares issued under employee stock purchase plan		2.1				2.1
Exercise of stock options		9.2				9.2
Exercise of stock options (in shares)	0.2
Share-based compensation		13.9				13.9
Balance at Mar. 31, 2018	$ 0.7	1,879.5	(21.7)	(2,579.2)	3,090.1	2,369.4
Balance (in shares) at Mar. 31, 2018	70.1
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					(494.6)	(494.6)
Unrealized gain (loss) on available-for-sale securities			2.6			2.6
Defined benefit pension plan			0.1			0.1
Shares issued under employee stock purchase plan		2.2				2.2
Restricted stock units withheld for taxes		(1.9)				(1.9)
Exercise of stock options		8.8				8.8
Exercise of stock options (in shares)	0.2
Share-based compensation		18.3				18.3
Cumulative effect of accounting change					(5.1)	(5.1)
Balance at Mar. 31, 2019	$ 0.7	$ 1,967.6	$ (5.2)	$ (2,579.2)	$ 2,935.1	$ 2,319.0
Balance (in shares) at Mar. 31, 2019	70.4