Supplemental financial information - Amounts Reclassified out of AOCI (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
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]	$ 12	$ 18
Tax Effect		(3)	(6)
Recognized within Net income, net of taxes		9	12
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]	(1)	(1)
Tax Effect		0	0
Recognized within Net income, net of taxes		$ (1)	$ (1)

[1] Detailed in Note 6.