Natural Gas And Oil Producing Activities (Capitalized Costs Relating To Oil And Gas Producing Activities Disclosure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Capitalized Costs Relating to Oil and Gas Producing Activities [Line Items]
Proved properties	$ 18,751	$ 15,860
Unproved properties	3,727	4,646
Total capitalized costs	22,478	20,506
Less: Accumulated depreciation, depletion and amortization	(16,248)	(8,327)
Net capitalized costs	6,230	12,179
Unproved property acquisition costs	692	3,934	$ 168
Capitalized Costs [Member] | Exploration Program in Canada [Member]
Net Capitalized Costs Relating to Oil and Gas Producing Activities [Line Items]
Unproved property acquisition costs	$ 50	$ 76