Earnings Per Share Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share Reconciliation [Abstract]
Net income available to common stockholders for purposes of computing basic earnings per share	$ 129.2	$ 120.5	$ 410.1	$ 348.3
Net income available to common stockholders for purposes of computing diluted earnings per share	$ 129.2	$ 120.5	$ 410.1	$ 348.3
Weighted-Average Number of Shares Outstanding Reconciliation [Abstract]
Basic shares	104,324	107,621	105,297	107,800
Effect of dilution	354	191	285	199
Diluted	104,678	107,812	105,582	107,999
Earnings Per Share [Abstract]
Basic earnings per share	$ 1.24	$ 1.12	$ 3.89	$ 3.23
Diluted earnings per share	$ 1.23	$ 1.12	$ 3.88	$ 3.23
Stock Options [Member]
Potentially Dilutive Shares Excluded from the Calculation [Abstract]
Stock options excluded as their inclusion would be anti-dilutive	14	34	159	220