Capital Stock (Summary of Cash Dividend Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Capital [Abstract]
Declaration date	Jul. 25, 2017	Apr. 27, 2017	Feb. 22, 2017	Jul. 26, 2016	Apr. 26, 2016	Feb. 18, 2016
Record date	Sep. 08, 2017	Jun. 09, 2017	Mar. 10, 2017	Sep. 09, 2016	Jun. 10, 2016	Mar. 10, 2016
Payment date	Sep. 25, 2017	Jun. 23, 2017	Mar. 24, 2017	Sep. 26, 2016	Jun. 24, 2016	Mar. 25, 2016
Cash dividends per share	$ 0.70	$ 0.65	$ 0.65	$ 0.65	$ 0.65	$ 0.65
Total payment	$ 181.4	$ 171.8	$ 172.2	$ 171.1	$ 170.9	$ 170.7	$ 525.4	$ 512.7