Derivative Instruments and Hedging Activities Derivative Instruments and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($)	Jun. 30, 2018	Sep. 30, 2017
Offsetting Derivative Assets and Liabilities [Abstract]
Collateral Already Posted, Aggregate Fair Value	$ 0	$ 0
Derivative assets	81,000,000	91,000,000
Gross amount eligible for offsetting, derivative assets	(18,000,000)	(16,000,000)
Net derivative amount, derivative assets	63,000,000	75,000,000
Derivative liabilities	2,948,000,000	2,105,000,000
Gross amount eligible for offsetting, derivative liabilities	(18,000,000)	(16,000,000)
Net derivative amount, derivative liabilities	$ 2,930,000,000	$ 2,089,000,000