Equity - Stock options (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Stock options, Number of Options
Outstanding at the beginning of the period (in shares)	2,055
Granted (in shares)	336	247
Exercised (in shares)	(79)
Cancelled (in shares)	(3)
Outstanding at the end of the period (in shares)	2,309		2,055
Exercisable at the end of the period (in shares)	1,757
Stock options, Weighted Average Exercise Price per Share
Outstanding at the beginning of the period (in dollars per share)	$ 84.36
Granted (in dollars per share)	88.35
Exercised (in dollars per share)	43.74
Cancelled (in dollars per share)	28.75
Outstanding at the end of the period (in dollars per share)	86.39		$ 84.36
Exercisable at the end of the period (in dollars per share)	$ 83.88
Additional information pertaining to stock options
Average Remaining Contractual Term, Outstanding	5 years 10 months 6 days		5 years 3 months 18 days
Average Remaining Contractual Term, Exercisable	4 years 9 months 21 days
Aggregate Intrinsic Value, Outstanding (in dollars)	$ 11		$ 34
Aggregate Intrinsic Value, Exercisable (in dollars)	11
Cash received from exercise of stock options	$ 4
Weighted average grant date fair value of stock options granted (per share)	$ 11.48	$ 14.02
Total intrinsic value of stock options exercised	$ 4	$ 4
Stock options
Share-based compensation
Term of award	10 years
Period of vesting	3 years
Required service period	1 year
Assumptions used to measure the fair value of awards
Expected life	5 years 6 months	5 years 6 months
Risk-free interest rate (as a percent)	1.40%	2.50%
Expected volatility (as a percent)	19.80%	19.70%
Expected dividend yield (as a percent)	2.90%	2.70%
Additional information pertaining to stock options
Unrecognized compensation cost	$ 5
Weighted-average period for amortization of unrecognized compensation cost	2 years 4 months 24 days