Acquisitions and Dispositions Acquisitions and Dispositions - Dispositions (Narrative) (Details) $ in Millions			3 Months Ended				6 Months Ended	9 Months Ended
	Oct. 10, 2016 USD ($) MW	Apr. 02, 2015 USD ($) MW	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset Impairment Charges				$ 10		$ 111		$ 208	$ 111
Proceeds from sale of the Midwest Generation Disposal Group									2,792
International Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset Impairment Charges					$ 194
International Disposal Group [Member] | Scenario, Forecast [Member] | International Energy [Member] | Minimum [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses							$ 1,700
Asset Impairment Charges			$ 325
International Disposal Group [Member] | Scenario, Forecast [Member] | International Energy [Member] | Maximum [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Divestiture of Businesses							$ 1,900
Asset Impairment Charges			$ 375
Midwest Generation Business [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax benefit				122		(1)		122	29
Midwest Generation Business [Member] | Duke Energy Ohio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Owned Average MW Capacity | MW		5,900
Proceeds from sale of the Midwest Generation Disposal Group		$ 2,800
Income tax benefit				$ 34		$ (2)		$ 34	$ 25
Subsequent Event [Member] | DEI Brazil Holdings [Member] | International Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Owned Average MW Capacity | MW	2,090
Enterprise Value	$ 1,200
Acquirer Termination Fee	$ 49
Subsequent Event [Member] | DEI Latin American [Member] | International Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Owned Average MW Capacity | MW	2,230
Enterprise Value	$ 1,200
Acquirer Termination Fee	$ 89