Supplemental financial information - Accumulated other comprehensive income (loss), net of taxes (AOCI) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	$ 8,907	$ 8,994
AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	(347)	(473)	$ (384)	$ (526)
Net actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	(347)	(473)
Prior service credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	2	2
Cash flow hedge derivative instruments - post adoption
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	$ (2)
Cash flow hedge derivative instruments - pre adoption
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total		$ (2)