Supplementary Information (Net Change in Non-Cash Working Capital) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Information [Abstract]
Accounts receivable and accrued revenues	$ (21)	$ 86	$ 314
Accounts payable and accrued liabilities	(226)	(233)	(14)
Income tax payable and receivable	(6)	(40)	(38)
Net change in non-cash working capital	$ (253)	$ (187)	$ 262