Acquisitions and Divestitures - Assets Held for Sale (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset impairment charge	$ 12	$ 66
Detroit Properties and Airplanes | Assets held for sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset impairment charge			$ 49	$ 49
Detroit Properties and Airplanes | Assets held for sale | Corporate-related
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset impairment charge				10
Detroit Properties and Airplanes | Assets held for sale | Americas
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset impairment charge				39
Detroit Airplane | Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration for disposal group			$ 36	$ 36
Detroit Properties and Airplane | Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration for disposal group					$ 35