Supplemental Oil and Gas Disclosures (Cost Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities Disclosure) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) $ / ft³	Dec. 31, 2016 USD ($) $ / ft³	Dec. 31, 2015 USD ($) $ / ft³
Supplemental Oil and Gas Disclosures [Abstract]
Proved property acquisition costs			$ 81
Unproved property acquisition costs	$ 194	$ 171	692
Exploration costs	22	17	50
Development costs	1,024	433	1,417
Capitalized costs incurred	$ 1,240	$ 621	$ 2,240
Full cost pool amortization per Mcfe | $ / ft³	0.45	0.38	1.00