Property, Plant And Equipment, Net (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Internal Costs Capitalized	$ 208	$ 161
Property, Plant and Equipment, Net	8,954	8,139
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	38,056	38,863
Impairments	0	1,396	$ 6,473
Bow Office Building [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	$ 1,255	1,194
Term of Lease Agreement Liability, In Years	25 years
Property, Plant and Equipment, Useful Life	60 years
Assets Held under Capital Leases [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	$ 46	51
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	684	648
Corporate, Non-segment [Member]
Property, Plant and Equipment [Line Items]
Fully Amortized Property Costs	63	58
Property, Plant and Equipment, Net	1,535	1,485
Canadian Operations [Member] | Operating Segments [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	862	602
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	14,047	12,896
Impairments		493	0
USA Operations [Member] | Operating Segments [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	6,555	6,050
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	$ 23,240	25,300
Impairments		$ 903	$ 6,473