Employee benefit plans - Net loss and prior service credit balances (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pensions Plans
Employee benefit plans
Net actuarial (loss) gain	$ (63)	$ (54)	$ (89)
Total recognized in accumulated other comprehensive loss (income)	(63)	(54)	(89)
Postretirement Benefit Plans
Employee benefit plans
Net actuarial (loss) gain	17	12	3
Net prior service credit	1	1	3
Total recognized in accumulated other comprehensive loss (income)	$ 18	$ 13	$ 6