Income Taxes (Deferred Income Taxes Liability By Jurisdiction Reflected In Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Income Taxes Assets	$ 1,043	$ 1,658
Deferred Income Tax Liabilities	(34)	(31)
Deferred Tax Assets, Net of Liabilities	1,009	1,627
Canada [Member]
Deferred Income Taxes Assets	555	568
Deferred Income Tax Liabilities	(34)	(31)
United States [Member]
Deferred Income Taxes Assets	$ 488	$ 1,090