Supplemental financial information - Amounts Reclassified out of AOCI (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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]	$ 83	$ 76	$ 78
Tax Effect		(27)	(25)	(25)
Recognized within Net income, net of taxes		56	51	53
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]	(6)	(5)	0
Tax Effect		1	2	0
Recognized within Net income, net of taxes		(5)	(3)	0
Derivative instruments
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax		1	1	2
Tax Effect		0	0	(1)
Recognized within Net income, net of taxes		$ 1	$ 1	$ 1

[1] Detailed in Note 10.