Acquisitions, Investments and Dispositions - Pike County Light & Power Company (Details) - Pike - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Sep. 30, 2015	Aug. 31, 2016	Dec. 31, 2015
Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from divestiture of business		$ 15
Discontinued Operations, Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment charge on assets held for sale	$ 5		$ 5
Impairment of long-lived assets to be disposed of, net of tax	$ 3
Assets held for sale			23
Liabilities held for sale			$ 5
O&R | Discontinued Operations, Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Period for transition services (up to)		18 months
Agreement to purchase and sell, term		3 years
Agreement to purchase and sell, term of extension option		3 years