Shareholders' Equity - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	$ 2	$ 2
Amount Reclassified from Accumulated Other Comprehensive Income | Employee Benefit Related Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs	(9)	(9)
Actuarial losses	10	11
Total before tax	(1)	(2)
Tax expense (benefit)	0	0
Total reclassifications for the period	$ 1	$ 2