Income taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income taxes
Canadian	$ (97)	$ 25	$ 60
Non-Canadian	(1,231)	(129)	7
(Loss) income before taxes	(1,328)	(104)	67
Current tax (benefit) expense:
Canadian	(5)	1	2
Non-Canadian		1
Total	(5)	2	2
Deferred tax benefit:
Canadian	(54)	(22)	(3)
Non-Canadian	(3)	(142)
Total	(57)	(164)	(3)
Income tax benefit	$ (62)	$ (162)	$ (1)