Supplemental cash flow - (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental cash flow information:
Cash paid for interest	$ (205)	$ (193)	$ (152)
Supplemental non-cash investing and financing activities:
Accrued capital expenditures	$ 13	$ 19	$ 19