Shareholders' Equity - Reclassifications from AOCI (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	$ 2	$ 10
Amount Reclassified from Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	2
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs	(9)	(1)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	11	11
Amount Reclassified from Accumulated Other Comprehensive Income | Employee Benefit Related Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	2	10
Tax expense (benefit)	$ 0	0
Total reclassifications for the period		$ 10