Earnings Per Share Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share Reconciliation [Abstract]
Net income available to common stockholders for purposes of computing basic earnings per share	$ 134.4	$ 120.1	$ 280.9	$ 227.8
Net income available to common stockholders for purposes of computing diluted earnings per share	$ 134.4	$ 120.1	$ 280.9	$ 227.8
Weighted-Average Number of Shares Outstanding Reconciliation [Abstract]
Basic shares	105,473	107,720	105,792	107,891
Effect of dilution	285	187	250	203
Diluted	105,758	107,907	106,042	108,094
Earnings Per Share [Abstract]
Basic earnings per share	$ 1.27	$ 1.12	$ 2.66	$ 2.11
Diluted earnings per share	$ 1.27	$ 1.11	$ 2.65	$ 2.11
Stock Options [Member]
Potentially Dilutive Shares Excluded from the Calculation [Abstract]
Stock options excluded as their inclusion would be anti-dilutive	111	48	210	185