Equity - Restricted stock units (Details) - Restricted stock units (RSUs) $ / shares in Units, shares in Thousands, $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares
Share-based compensation
Vesting terms	3 years
Service period over which compensation expense would be amortized	1 year
Restricted stock unit activity
Non-vested at the beginning of the period (in shares) | shares	344
Granted (in shares) | shares	174
Vested (in shares) | shares	(136)
Cancelled (in shares) | shares	(37)
Non-vested at the end of the period (in shares) | shares	345
Weighted-average fair value per share
Non-vested at the beginning of the period (in dollars per share) | $ / shares	$ 115.06
Granted (in dollars per share) | $ / shares	91.28
Vested (in dollars per share) | $ / shares	101.40
Cancelled (in dollars per share) | $ / shares	115.79
Non-vested at the end of the period (in dollars per share) | $ / shares	$ 108.37
Other disclosures
Unrecognized compensation cost | $	$ 17
Weighted-average period for amortization of unrecognized compensation cost	1 year 10 months 24 days