Stock-based compensation (Summary of Stock Options) (Details) - Stock options [Member] $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Options
Balance unexercised at beginning of period (in shares) | shares	3.1
Granted (in shares) | shares	1.5
Exercised (in shares) | shares	(0.5)
Expired/forfeited (in shares) | shares	(0.1)
Balance unexercised at end of period (in shares) | shares	4.0
Vested or expected to vest (in shares) | shares	3.7
Exercisable (in shares) | shares	1.3
Weighted- average exercise price
Balance unexercised at beginning of period (in usd per share) | $ / shares	$ 100.21
Granted (in usd per share) | $ / shares	162.60
Exercised (in usd per share) | $ / shares	57.24
Expired/forfeited (in usd per share) | $ / shares	159.13
Balance unexercised at end of period (in shares) | $ / shares	127.08
Weighted-average exercise price, vested or expected to vest (in usd per share) | $ / shares	124.84
Weighted-average exercise price, exercisable (in usd per share) | $ / shares	$ 58.23
Stock options information [Abstract]
Weighted-average remaining contractual life (years), unexercised	6 years 10 months 24 days
Weighted-average remaining contractual life (years), vested or expected to vest	6 years 9 months 18 days
Weighted-average remaining contractual life (years), exercisable	2 years 9 months 18 days
Aggregate intrinsic value, unexercised | $	$ 186
Aggregate intrinsic value, vested or expected to vest | $	182
Aggregate intrinsic value, exercisable | $	$ 147