Income Taxes (Provision For Income Taxes) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current Tax									$ (63)	$ (78)	$ (34)
Deferred Tax									666	(598)	(2,811)
Income Tax Expense (Recovery), Total	$ 376	$ 228	$ (4)	$ 3	$ 30	$ 62	$ (467)	$ (301)	603	(676)	(2,845)
Canada [Member]
Current Tax									(59)	(82)	(25)
Deferred Tax									55	(163)	(316)
United States [Member]
Current Tax									(9)		(17)
Deferred Tax									611	(435)	(2,495)
Other Countries [Member]
Current Tax									$ 5	$ 4	$ 8