Equity Items - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Equity [Abstract]
Unrealized translation gain (loss), before tax	$ (52)	$ (9)	$ (14)	$ (40)
Unrealized translation gain (loss), net of tax	(52)	(9)	(14)	(40)
Adjustment of unrecognized prior service cost (credit), before tax				(7)
Adjustment of unrecognized prior service cost (credit), tax				1
Adjustment of unrecognized prior service cost (credit), net of tax				(6)
Total other comprehensive income (loss), before tax	(52)	(9)	(14)	(47)
Total other comprehensive income (loss), tax				1
Other comprehensive income (loss)	$ (52)	$ (9)	$ (14)	$ (46)