Acquisitions, Discontinued Operations and Dispositions - Summarized of Results of Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on disposal of discontinued operations, net of tax			$ 0	$ (22)
Gain/(Loss) from discontinued operations, net of tax	$ 13	$ 69	401	64
Discontinued Operations | South Central Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	0	107	31	209
Operating costs and expenses	0	(91)	(23)	(177)
Gain from discontinued operations, net of tax	0	16	8	32
Gain on disposal of discontinued operations, net of tax	1	0	28	0
Gain/(Loss) from discontinued operations, net of tax	1	16	36	32
Discontinued Operations | Carlsbad Energy Holdings LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	0	368	19	628
Operating costs and expenses	0	(223)	(9)	(453)
Other expenses	0	(65)	(5)	(123)
Gain/(loss) from operations of discontinued components, before tax	0	80	5	52
Income tax expense	0	2	0	(5)
Gain from discontinued operations, net of tax	0	78	5	57
Gain on disposal of discontinued operations, net of tax	(17)	0	331	0
Other Commitments, Indemnification and Fees	27	0	27	0
Gain on disposal of discontinued operations, net of tax	10	0	358	0
Gain/(Loss) from discontinued operations, net of tax	$ 10	$ 78	$ 363	$ 57