Accumulated Other Comprehensive (Loss) Income - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax expense (benefits) associated with reclassification adjustments	$ 9,000,000	$ 76,000,000	$ 58,000,000	$ 164,000,000
Income tax benefit recorded for losses recognized in other comprehensive income relating to cash flow hedges	0	0	(5,000,000)	0
Benefit Plans Adjustments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax expense (benefits) associated with reclassification adjustments	$ (6,000,000)	$ (4,000,000)	$ (12,000,000)	$ (9,000,000)