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Stock-based compensation (Summary of Stock Options) (Details) - Stock options [Member]
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
$ / shares
shares
Options  
Balance unexercised at beginning of period (in shares) | shares 4.8
Granted (in shares) | shares 1.0
Exercised (in shares) | shares (0.9)
Expired/forfeited (in shares) | shares (0.2)
Balance unexercised at end of period (in shares) | shares 4.7
Vested or expected to vest (in shares) | shares 4.5
Exercisable (in shares) | shares 1.5
Weighted- average exercise price  
Balance unexercised at beginning of period (in usd per share) | $ / shares $ 157.00
Granted (in usd per share) | $ / shares 236.36
Exercised (in usd per share) | $ / shares 117.90
Expired/forfeited (in usd per share) | $ / shares 178.36
Balance unexercised at end of period (in shares) | $ / shares 179.90
Weighted-average exercise price, vested or expected to vest (in usd per share) | $ / shares 178.37
Weighted-average exercise price, exercisable (in usd per share) | $ / shares $ 150.80
Stock options information [Abstract]  
Weighted-average remaining contractual life (years), unexercised 7 years 3 months 18 days
Weighted-average remaining contractual life (years), vested or expected to vest 7 years 3 months 18 days
Weighted-average remaining contractual life (years), exercisable 5 years 6 months
Aggregate intrinsic value, unexercised | $ $ 243
Aggregate intrinsic value, vested or expected to vest | $ 239
Aggregate intrinsic value, exercisable | $ $ 120