Property, Plant And Equipment, Net (Schedule Of Property, Plant And Equipment) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	$ 47,010	$ 47,002
Less: Accumulated depreciation, depletion and amortization	(38,056)	(38,863)
Property, plant and equipment, net	8,954	8,139
Operating Segments [Member] | Canadian Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	14,909	13,498
Less: Accumulated depreciation, depletion and amortization	(14,047)	(12,896)
Property, plant and equipment, net	862	602
Operating Segments [Member] | USA Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	29,795	31,350
Less: Accumulated depreciation, depletion and amortization	(23,240)	(25,300)
Property, plant and equipment, net	6,555	6,050
Operating Segments [Member] | Market Optimization [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	2	2
Corporate, Non-segment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, net	1,535	1,485
Proved Properties [Member] | Operating Segments [Member] | Canadian Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	14,555	13,159
Less: Accumulated depreciation, depletion and amortization	(14,047)	(12,896)
Property, plant and equipment, net	508	263
Proved Properties [Member] | Operating Segments [Member] | USA Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	25,610	26,393
Less: Accumulated depreciation, depletion and amortization	(23,240)	(25,300)
Property, plant and equipment, net	2,370	1,093
Unproved Properties [Member] | Operating Segments [Member] | Canadian Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	311	285
Property, plant and equipment, net	311	285
Unproved Properties [Member] | Operating Segments [Member] | USA Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	4,169	4,913
Property, plant and equipment, net	4,169	4,913
Other Capitalized Property Plant and Equipment [Member] | Operating Segments [Member] | Canadian Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	43	54
Property, plant and equipment, net	43	54
Other Capitalized Property Plant and Equipment [Member] | Operating Segments [Member] | USA Operations [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	16	44
Property, plant and equipment, net	16	44
Other Capitalized Property Plant and Equipment [Member] | Operating Segments [Member] | Market Optimization [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	7	6
Less: Accumulated depreciation, depletion and amortization	(5)	(4)
Property, plant and equipment, net	2	2
Other Capitalized Property Plant and Equipment [Member] | Corporate, Non-segment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Cost	2,299	2,148
Less: Accumulated depreciation, depletion and amortization	(764)	(663)
Property, plant and equipment, net	$ 1,535	$ 1,485