CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 430	$ 1,707	$ 2,500	$ 3,648
Investment securities, available-for-sale:
Net unrealized gain	19	26	16	60
Income tax effect	(7)	(7)	(8)	(23)
Reclassification adjustment for net loss (gain) realized in net income	(1)	3	(1)	3
Income tax effect	0	1	0	1
Defined benefit pension and other postretirement plans:
Net unrealized actuarial (loss) gain and prior service credit	(5)	5	(5)	61
Income tax effect	2	(2)	2	(23)
Amortization of actuarial loss and prior service credit realized in net income	15	2	21	(5)
Income tax effect	(7)	0	(9)	2
Derivative instruments classified as cash flow hedges:
Net unrealized (loss) gain	(49)	(54)	25	(38)
Income tax effect	11	11	4	6
Reclassification adjustment for net loss (gain) realized in net income	8	(37)	20	(85)
Income tax effect	(3)	11	(5)	25
Foreign currency translation adjustments	404	0	(584)	0
Other comprehensive income (loss), net of tax	389	(47)	(522)	(22)
Comprehensive income	$ 819	$ 1,660	$ 1,978	$ 3,626