Natural Gas And Oil Producing Activities (Cost Incurred In Oil And Gas Property Acquisition, Exploration, And Development Activities Disclosure) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) $ / ft³	Dec. 31, 2014 USD ($) $ / ft³	Dec. 31, 2013 USD ($) $ / ft³
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Proved property acquisition costs	$ 81	$ 1,455	$ 1
Unproved property acquisition costs	692	3,934	168
Exploration costs	50	232	192
Development costs	1,417	1,600	1,662
Capitalized costs incurred	$ 2,240	$ 7,221	$ 2,023
Full cost pool amortization per Mcfe | $ / ft³	1.00	1.10	1.08
Costs Incurred in Natural Gas and Oil Exploration and Development [Member] | Exploration Program in Canada [Member]
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]
Unproved property acquisition costs		$ 1	$ 17
Exploration costs		$ 3	$ 12