Additional Shareholders' Equity Information - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification to retained earnings	$ 6
Deferred losses	(10,484)	$ (9,055)	$ (10,619)	$ (9,600)
Interest Rate Cash Flow Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred losses	$ 23	22
Interest Rate Cash Flow Hedge | Interest Rate Contract
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred losses		$ 22