Note 9. Derivative Instruments and Hedging Activities Derivative Assets and Liabilities Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 8	$ 49
Derivative Liability, Fair Value, Gross Liability	1,191	1,158
Derivative Asset, Collateral, Obligation to Return Cash, Offset	(2)	(1)
Derivative Liability, Collateral, Right to Reclaim Cash, Offset	(2)	(1)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	6	48
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 1,189	$ 1,157