Derivative Instruments and Hedging Activities Derivative Instruments and Hedging Activities - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016
Offsetting Derivative Assets and Liabilities [Abstract]
Derivative assets	$ 32	$ 95
Derivative liabilities	1,608	2,980
Gross amount eligible for offsetting, derivative assets	(16)	(21)
Gross amount eligible for offsetting, derivative liabilities	(16)	(21)
Net derivative amount, derivative assets	16	74
Net derivative amount, derivative liabilities	$ 1,592	$ 2,959