CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2014	$ 2,101.9	$ 292.8	$ (789.5)	$ 218.4	$ 2,414.9	$ (34.7)
Beginning Balance (in shares) at Dec. 31, 2014		292.8	(26.2)
Net Income	410.4	$ 0.0	$ 0.0	0.0	410.4	0.0
Other comprehensive (loss) income	(11.2)	0.0	0.0	0.0	0.0	(11.2)
Cash dividends	(175.3)	0.0	0.0	0.0	(175.3)	0.0
Stock purchases	(363.1)	$ 0.0	$ (363.1)	0.0	0.0	0.0
Stock purchases (in shares)		0.0	(8.8)
Transfer of stock for settlement of share repurchase agreement	0.0	$ 0.0	$ 4.1	(4.1)	0.0	0.0
Transfer of stock for settlement of share repurchase agreement (in shares)		0.0	0.2
Stock based compensation expense and stock option plan transactions, including related income tax benefits	60.5	$ 0.0	$ 44.8	15.7	0.0	0.0
Stock based compensation expense and stock option plan transactions, including related income tax benefits (in shares)		0.0	2.0
Ending Balance at Dec. 31, 2015	2,023.2	$ 292.8	$ (1,103.7)	230.0	2,650.0	(45.9)
Ending Balance (in shares) at Dec. 31, 2015		292.8	(32.8)
Net Income	459.0	$ 0.0	$ 0.0	0.0	459.0	0.0
Other comprehensive (loss) income	(17.9)	0.0	0.0	0.0	0.0	(17.9)
Cash dividends	(183.0)	0.0	0.0	0.0	(183.0)	0.0
Stock purchases	(400.0)	$ 0.0	$ (400.0)	0.0	0.0	0.0
Stock purchases (in shares)		0.0	(9.0)
Stock based compensation expense and stock option plan transactions, including related income tax benefits	96.6	$ 0.0	$ 75.2	21.4	0.0	0.0
Stock based compensation expense and stock option plan transactions, including related income tax benefits (in shares)		0.0	2.9
Ending Balance at Dec. 31, 2016	1,977.9	$ 292.8	$ (1,428.5)	251.4	2,926.0	(63.8)
Ending Balance (in shares) at Dec. 31, 2016		292.8	(38.9)
Net Income	743.4	$ 0.0	$ 0.0	0.0	743.4	0.0
Other comprehensive (loss) income	27.4	0.0	0.0	0.0	0.0	27.4
Cash dividends	(190.4)	0.0	0.0	0.0	(190.4)	0.0
Stock purchases	$ (400.0)	$ 0.0	$ (400.0)	0.0	0.0	0.0
Stock purchases (in shares)	(8.2)	0.0	(8.2)
Stock based compensation expense and stock option plan transactions, including related income tax benefits	$ 59.7	$ 0.0	$ 46.5	13.2	0.0	0.0
Stock based compensation expense and stock option plan transactions, including related income tax benefits (in shares)		0.0	1.9
Ending Balance at Dec. 31, 2017	$ 2,218.0	$ 292.8	$ (1,782.0)	$ 264.6	$ 3,479.0	$ (36.4)
Ending Balance (in shares) at Dec. 31, 2017		292.8	(45.2)