Exit Activities and Discontinued Operations - Income from Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax expense	$ 9	$ 4	$ 25	$ 10
Income from discontinued operations, net of tax	30	12	82	34
Discontinued Operations, Held-for-sale | EnerBank
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenue	70	68	209	191
Operating expenses	17	39	60	104
Interest expense	11	13	34	43
Income before income taxes	42	16	115	44
Transaction costs	(3)	0	(8)	0
Income from discontinued operations before income taxes	39	16	107	44
Income tax expense	9	4	25	10
Income from discontinued operations, net of tax	$ 30	$ 12	$ 82	$ 34