Supplemental financial information - Amounts reclassified out of accumulated other comprehensive income (loss), net of taxes, to net income (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net actuarial losses of defined benefit plans
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax	[1]	$ 65	$ 83	$ 76
Tax Effect		(15)	(27)	(25)
Recognized within net income, net of taxes		50	56	51
Prior service credit of defined benefit plans
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax	[1]	(4)	(6)	(5)
Tax Effect		1	1	2
Recognized within net income, net of taxes		(3)	(5)	(3)
Derivative instruments
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax		0	1	1
Tax Effect		0	0	0
Recognized within net income, net of taxes		$ 0	$ 1	$ 1

[1] Detailed in Note 9.