Capital Stock (Summary of Stock Option Activity) (Details) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares, Options outstanding at January 1 (in shares) | shares	14.9
Shares, Granted (in shares) | shares	2.9
Shares, Exercised (in shares) | shares	(2.5)
Shares, Cancelled (in shares) | shares	(0.6)
Shares, Options outstanding at December 31 (in shares) | shares	14.7
Shares, Exercisable at December 31 (in shares) | shares	7.5
Shares, Options expected to vest (in shares) | shares	6.6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted average exercise price, Options outstanding at January 1 (in dollars per share) | $ / shares	$ 73.14
Weighted average exercise price, Granted (in dollars per share) | $ / shares	122.66
Weighted average exercise price, Exercised (in dollars per share) | $ / shares	62.66
Weighted average exercise price, Cancelled (in dollars per share) | $ / shares	97.87
Weighted average exercise price, Options outstanding at December 31 (in dollars per share) | $ / shares	83.71
Weighted average exercise price, Exercisable at December 31 (in dollars per share) | $ / shares	65.47
Weighted average exercise price, Options expected to vest (in dollars per share) | $ / shares	$ 101.83
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average remaining contractual term, Options outstanding at December 31 (in years)	5 years 5 months
Weighted-average remaining contractual term, Exercisable at December 31 (in years)	4 years 2 months
Weighted-average remaining contractual term, Options expected to vest (in years)	7 years 11 months
Aggregate intrinsic value, Options outstanding at December 31 | $	$ 956.5
Aggregate intrinsic value, Exercisable at December 31 | $	666.8
Aggregate intrinsic value, Options expected to vest | $	$ 347.9