Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Income Tax Contingency [Line Items]
Income tax provision (benefit)	$ 64		$ (28)	$ 74	$ 237
Effective tax rate (as percent)	103.00%		16.00%	180.00%	232.00%
Federal statutory income tax rate (as percent)				12.50%
Change in valuation allowance on deferred tax asset	$ 43
Unrecognized tax benefits	280			$ 280
Unrecognized tax benefits that would impact effective tax rate	131			131
Net accrued interest	8			$ 8
Change in valuation allowance		$ 100
Impairment charges on long-lived assets held-for-use	66
Income tax benefit associated with impairment charges of long-lived asset	2
Goodwill impairment charge	$ 299		$ 299
Income tax benefit associated with goodwill impairment charges			$ 20		$ 20
Foreign Tax Authority | GAAS
Income Tax Contingency [Line Items]
Income tax benefit on reduction of deferred tax liability						$ 7
Foreign income tax rate differential (as percent)		15.00%				25.00%