Derivative Instruments and Hedging Activities - Derivative Assets and Liabilities Offsetting (Details) - USD ($)	Mar. 31, 2019	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Cash collateral pledged	$ 0	$ 0
Cash collateral received	0	0
Gross amount recognized, asset	29,000,000	23,000,000
Gross amount eligible for offsetting, asset	(5,000,000)	(5,000,000)
Net amount, asset	24,000,000	18,000,000
Gross amount recognized, liability	1,138,000,000	1,177,000,000
Gross amount eligible for offsetting, liability	(5,000,000)	(5,000,000)
Net amount, liability	$ 1,133,000,000	$ 1,172,000,000