Property, plant and equipment - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) building	Dec. 31, 2020 USD ($) property	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 property
Property, plant and equipment
Reduction of gain on sale leaseback			$ 1	$ (136)	$ (33)
Gain on sale of assets			(1)	$ 136	$ 33
Properties in Palo Alto, California
Property, plant and equipment
Net proceeds	$ 280
Reduction of gain on sale leaseback	$ (136)		$ 4
Number of properties | property			1			1
Number of buildings | building		1
Proceeds from sale of building		$ 68
Gain on sale of assets		$ 33