Exit Activities and Discontinued Operations - Income from Discontinued Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax expense	$ 7	$ 3	$ 16	$ 6
Income from discontinued operations, net of tax	18	8	52	22
Discontinued Operations, Held-for-sale | EnerBank
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenue	69	61	139	123
Operating expenses	28	36	43	65
Interest expense	11	14	23	30
Income before income taxes	30	11	73	28
Transaction costs	(5)	0	(5)	0
Income from discontinued operations before income taxes	25	11	68	28
Income tax expense	7	3	16	6
Income from discontinued operations, net of tax	$ 18	$ 8	$ 52	$ 22