Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Tax Contingency [Line Items]
Effective tax rate (as percent)	48.00%	384.00%
Statutory rate (as percent)	12.50%	12.50%
Unrecognized tax benefits	$ 283
Unrecognized tax benefits that would impact effective tax rate	132
Net accrued interest	$ 6
Non-cash estimated tax expense			$ 100
Foreign tax authority | GAAS
Income Tax Contingency [Line Items]
Foreign tax rate differential (as percent)	15.00%		25.00%
Foreign tax rate differential	$ 7