Capital Stock (Summary of Stock Option Activity) (Details) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Number of Shares [Roll Forward]
Outstanding at beginning of period, Number of Shares | shares	4.3
Granted, Number of Shares | shares	0.9
Exercised, Number of Shares | shares	(1.0)
Forfeited or expired, Number of Shares | shares	(0.2)
Outstanding at end of period, Number of Shares | shares	4.0
Exercisable at end of period, Number of Shares | shares	2.6
Weighted-Average Option Price Per Share [Roll Forward]
Outstanding at beginning of period, Weighted-Average Option Price per Share | $ / shares	$ 124.31
Granted, Weighted-Average Option Price per Share | $ / shares	232.61
Exercised, Weighted-Average Option Price per Share | $ / shares	110.85
Forfeited or expired, Weighted-Average Option Price per Share | $ / shares	187.56
Outstanding at end of period, Weighted-Average Option Price per Share | $ / shares	148.58
Exercisable at end of period, Weighted-Average Option Price per Share | $ / shares	$ 125.25
Outstanding at end of period, Weighted-Average Remaining Contractual Life	6 years 5 months 1 day
Exercisable at end of period, Weighted-Average Remaining Contractual Life	5 years 3 months 7 days
Outstanding at end of period, Aggregate Intrinsic Value | $	$ 498
Exercisable at end of period, Aggregate Intrinsic Value | $	$ 393