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, 2013	$ 0.6	$ 1,057.2	$ (13.1)	$ (513.4)	$ 728.1	$ 1,259.4
Balance (in shares) at Dec. 31, 2013	63.0
Increase (Decrease) in Stockholders' Equity
Net income					340.1	340.1
Foreign currency translation adjustments			(4.8)			(4.8)
Unrealized gain on available-for-sale securities			(0.3)			(0.3)
Defined benefit pension plan			1.5			1.5
Shares issued under employee stock purchase plan		3.3				3.3
Conversion of 2016 convertible notes	$ 0.1	192.9		(189.3)		3.7
Conversion of 2016 convertible notes (in shares)	1.5
Equity component - 2016 convertible notes		11.1				11.1
Repurchase of shares				(483.1)		(483.1)
Exercise of stock options		50.2				50.2
Exercise of stock options (in shares)	1.5
Tax benefit from exercises of non-qualified stock options		30.8				30.8
Share-based compensation		30.6				30.6
Balance at Dec. 31, 2014	$ 0.7	1,376.1	(16.7)	(1,185.8)	1,068.2	1,242.5
Balance (in shares) at Dec. 31, 2014	66.0
Increase (Decrease) in Stockholders' Equity
Net income					651.6	651.6
Foreign currency translation adjustments			(5.3)			(5.3)
Defined benefit pension plan			1.6			1.6
Shares issued under employee stock purchase plan		4.0				4.0
Conversion of 2016 convertible notes		324.7		(321.8)		2.9
Conversion of 2016 convertible notes (in shares)	2.0
Equity component - 2016 convertible notes		3.0				3.0
Repurchase of shares				(394.5)		(394.5)
Exercise of stock options		39.3				39.3
Exercise of stock options (in shares)	1.0
Tax benefit from exercises of non-qualified stock options		37.4				37.4
Share-based compensation		6.1				6.1
Balance at Dec. 31, 2015	$ 0.7	1,790.6	(20.4)	(1,902.1)	1,719.8	1,588.6
Balance (in shares) at Dec. 31, 2015	69.0
Increase (Decrease) in Stockholders' Equity
Net income					713.7	713.7
Foreign currency translation adjustments			(3.0)			(3.0)
Defined benefit pension plan			6.6			6.6
Shares issued under employee stock purchase plan		4.3				4.3
Conversion of 2016 convertible notes		7.6		(7.5)		0.1
Conversion of 2016 convertible notes (in shares)	0.1
Equity component - 2016 convertible notes		0.1				0.1
Shares issued upon expiration of warrants		(30.0)		30.0
Repurchase of shares				(500.0)		(500.0)
Exercise of stock options		7.7				7.7
Exercise of stock options (in shares)	0.2
Tax benefit from exercises of non-qualified stock options		5.9				5.9
Share-based compensation		27.3				27.3
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