Supplemental financial information - Accumulated other comprehensive income (loss), net of taxes (AOCI) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	$ 16,903	$ 16,897
AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	(140)	(205)	$ (254)	$ (157)
Net actuarial loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	(144)	(207)
Prior service credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	1	0
Unrealized gains on available-for-sale investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	2	2
Derivative instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total	$ 1	$ 0