Shareholders' Equity - Reclassifications from AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	$ 11	$ 9	$ 21	$ 18
Employee Benefit Related Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	11	9	21	18
Amount Reclassified from Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total reclassifications for the period	4		14
Amount Reclassified from Accumulated Other Comprehensive Income | Employee Benefit Related Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service costs			(1)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	11	9	22	18
Amount Reclassified from Accumulated Other Comprehensive Income | Employee Benefit Related Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	11	9	21	18
Tax expense (benefit)	$ (7)	0	$ (7)	0
Total reclassifications for the period		$ 9		$ 18