Income taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income taxes
Canadian	$ 77	$ (806)	$ (181)
Non-Canadian		(117)	(42)
Income (loss) before taxes	77	(923)	(223)
Current tax (benefit) expense:
Canadian	5		1
Total	5		1
Deferred tax benefit:
Canadian		(2)	(145)
Non-Canadian		(46)	(24)
Total		(48)	(169)
Income tax benefit	$ 5	$ (48)	$ (168)