Employee benefit plans - Combined Data for Defined Benefit Pension and Post Retirement Healthcare Plans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits | Domestic
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0	$ 0
Interest cost	39	44
Expected return on assets	80	84
Other	22	13
Net periodic benefit (credit) cost	(19)	(27)
Pension Benefits | Foreign
Defined Benefit Plan Disclosure [Line Items]
Service cost	3	3
Interest cost	7	8
Expected return on assets	10	11
Other	3	0
Net periodic benefit (credit) cost	3	0
Healthcare Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0
Interest cost	1	2
Expected return on assets	1	2
Other	(1)	(1)
Net periodic benefit (credit) cost	$ (1)	$ (1)