Derivative Instruments and Hedging Activities Derivative Instruments and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($)	Mar. 31, 2019	Sep. 30, 2018
Offsetting Derivative Assets and Liabilities [Abstract]
Collateral Already Posted, Aggregate Fair Value	$ 0	$ 0
Derivative assets	88,000,000	80,000,000
Gross amount eligible for offsetting, derivative assets	(10,000,000)	(12,000,000)
Net derivative amount, derivative assets	78,000,000	68,000,000
Derivative liabilities	3,085,000,000	3,175,000,000
Gross amount eligible for offsetting, derivative liabilities	(10,000,000)	(12,000,000)
Net derivative amount, derivative liabilities	$ 3,075,000,000	$ 3,163,000,000