Supplemental financial information - Accumulated other comprehensive income (loss), net of taxes (AOCI) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	$ 9,187	$ 8,907
AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	(360)	(347)	$ (473)	$ (384)
Net actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	(359)	(347)
Prior service credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	1	2
Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	$ (2)	$ (2)