Dispositions Dispositions (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended
	Apr. 30, 2016	Apr. 30, 2016	Mar. 31, 2016	Mar. 31, 2015
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment			$ 17	$ 2
Subsequent Event [Member] | Wyoming Upstream and Midstream Assets [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment	$ 870
Subsequent Event [Member] | Shenandoah in Gulf of Mexico, Piceance in Colorado, and West Texas [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Oil and Gas Property and Equipment		$ 80