Accumulated Other Comprehensive Income (Loss) - Balance Rollforward (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 8,700	$ 8,003
Current period change	(23)	13
Ending balance	8,747	8,357
Accumulated Other Comprehensive Income (Loss) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	41	(27)
Current period change	(23)	13
Ending balance	18	(14)
Investments Available-for-sale [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	58	(7)
Other comprehensive income (loss) before reclassifications	(24)	12
Ending balance	34	5
Cash Flow Hedging Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(17)	(20)
Amounts reclassified from accumulated other comprehensive income (loss)	1	1
Ending balance	$ (16)	$ (19)