Postretirement Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Postretirement Benefit Plans [Line Items]
Deferred actuarial gains in accumulated other comprehensive income	$ (1,161)
Gains (Losses) that will be amortized into earnings in 2018	(142)
Postretirement Plans [Member]
Postretirement Benefit Plans [Line Items]
Deferred actuarial gains in accumulated other comprehensive income	141
Gains (Losses) that will be amortized into earnings in 2018	$ 19
Discount rates used in measuring benefit obligations	3.45%	3.10%	3.80%
Health care cost trend rate assumed for next year	7.50%
Ultimate health care cost trend rate over subsequent 11 years	5.00%
Future health care benefit payments, 2018	$ 14
Future health care benefit payments, 2019	14
Future health care benefit payments, 2020	14
Future health care benefit payments, 2021	14
Future health care beneift payments, 2022	14
Total future health care benefit payments, 2023 - 2027	$ 60
Health care cost trend rate year	11 years