Accumulated Other Comprehensive Income (Loss) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Detail) - 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]
Amounts reclassified into income			$ 34
Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of losses into income	$ 22	$ 19	44	$ 37
Associated tax benefits, benefit plans	(7)	(6)	(14)	(13)
Amounts reclassified into income	15	12	29	24
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification of losses into income	2	5	8	9
Associated tax benefits, cash flow hedges	(1)	(2)	(3)	(3)
Amounts reclassified into income	$ 1	$ 3	$ 5	$ 6