Employee benefit plans - Summarizes the other changes in plan assets and benefit obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pensions Plans
Employee benefit plans
Net loss (gain)	$ 10	$ 8	$ 11
Amortization of net (loss) gain	(1)	(1)	(4)
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax, Total	9	7	7
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax	6	9	13
Postretirement Benefit Plans
Employee benefit plans
Net loss (gain)	(6)		(4)
Prior service credit		(1)
Amortization of prior service (cost) credit		2
Amortization of net (loss) gain	1	1
Curtailment loss		1	2
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, before Tax, Total	(5)	3	(2)
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax	$ (4)	$ (23)	$ (1)