Derivative Instruments and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Sep. 30, 2020
Offsetting Derivative Assets and Liabilities [Abstract]
Derivative assets	$ 48	$ 87
Gross amount eligible for offsetting, derivative assets	(8)	(10)
Net derivative amount, derivative assets	40	77
Derivative liabilities	3,022	3,020
Gross amount eligible for offsetting, derivative liabilities	(8)	(10)
Net derivative amount, derivative liabilities	$ 3,014	$ 3,010