Supplemental financial information - Amounts Reclassified out of AOCI (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net actuarial gains (losses) of defined benefit plans
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Reclassified from other comprehensive income (loss), before tax	[1]	$ 25	$ 17	$ 59	$ 57
Tax Effect		(8)	(5)	(19)	(18)
Recognized within Net income, net of taxes		17	12	40	39
Prior service (cost) 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)	(4)	(4)
Tax Effect		0	0	1	1
Recognized within Net income, net of taxes		$ (1)	$ (1)	$ (3)	$ (3)

[1] Detailed in Note 7.