Capital Stock (Summary of Stock Option Activity) (Details) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares
Number of Shares [Roll Forward]
Outstanding at beginning of period, Number of Shares | shares	5.6
Granted, Number of Shares | shares	1.1
Exercised, Number of Shares | shares	(1.8)
Forfeited or expired, Number of Shares | shares	(0.2)
Outstanding at end of period, Number of Shares | shares	4.7
Exercisable at end of period, Number of Shares | shares	3.0
Weighted-Average Option Price Per Share [Roll Forward]
Outstanding at beginning of period, Weighted-Average Option Price per Share | $ / shares	$ 102.80
Granted, Weighted-Average Option Price per Share | $ / shares	167.38
Exercised, Weighted-Average Option Price per Share | $ / shares	87.22
Forfeited or expired, Weighted-Average Option Price per Share | $ / shares	136.02
Outstanding at end of period, Weighted-Average Option Price per Share | $ / shares	122.55
Exercisable at end of period, Weighted-Average Option Price per Share | $ / shares	$ 106.20
Outstanding at end of period, Weighted-Average Remaining Contractual Life	6 years 3 months 7 days
Exercisable at end of period, Weighted-Average Remaining Contractual Life	4 years 10 months 20 days
Outstanding at end of period, Aggregate Intrinsic Value | $	$ 318.3
Exercisable at end of period, Aggregate Intrinsic Value | $	$ 253.8