Background and Basis of Presentation - Narrative (Details) $ in Millions	3 Months Ended				6 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)
Number of segments | segment					2
Quantifying Misstatement in Current Year Financial Statements, Amount | $	$ (672)	$ (54)	$ (13)	$ 9	$ (726)	$ (4)