Discontinued Operations and Dispositions - Summarized Results of Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain from discontinued operations, net of tax	$ 0	$ 388
Discontinued Operations, Disposed of by Sale | South Central Portfolio
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues		31
Operating costs and expenses		(23)
Gain from operations of discontinued components		8
Gain on disposal of discontinued operations, net of tax		27
Gain from discontinued operations, net of tax		35
Discontinued Operations, Disposed of by Sale | Carlsbad Energy Holdings LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues		19
Operating costs and expenses		(9)
Other expense		(5)
Gain from operations of discontinued components		5
Gain on disposal of discontinued operations, net of tax		348
Gain from discontinued operations, net of tax		$ 353