Capital Stock (Cash Dividend Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends, Cash [Abstract]
Declaration Date	Oct. 24, 2017	Jul. 25, 2017	Apr. 27, 2017	Feb. 22, 2017	Nov. 01, 2016	Jul. 26, 2016	Apr. 26, 2016	Feb. 18, 2016
Record Date	Dec. 05, 2017	Sep. 08, 2017	Jun. 09, 2017	Mar. 10, 2017	Dec. 05, 2016	Sep. 09, 2016	Jun. 10, 2016	Mar. 10, 2016
Payment Date	Dec. 21, 2017	Sep. 25, 2017	Jun. 23, 2017	Mar. 24, 2017	Dec. 21, 2016	Sep. 26, 2016	Jun. 24, 2016	Mar. 25, 2016
Cash Dividend per Share	$ 0.700	$ 0.7000	$ 0.6500	$ 0.6500	$ 0.6500	$ 0.6500	$ 0.6500	$ 0.6500
Total	$ 179.5	$ 181.4	$ 171.8	$ 172.2	$ 171.3	$ 171.1	$ 170.9	$ 170.7	$ 704.9	$ 684.0	$ 656.6