FAIR VALUE, Level 3 Financial Assets and Liabilities, Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Percentage of instruments measured at fair value on a recurring basis
Instruments measured at fair value, percentage of assets (in hundredths)	11.00%		11.00%		12.00%
Instruments measured at fair value, percentage of liabilities (in hundredths)	1.00%		1.00%		2.00%
Instruments measured at fair value, level 3, percentage of assets (in hundredths)	1.00%		1.00%		2.00%
Trading instruments | Trading instruments
Changes in Level 3 recurring fair value measurements, liabilities [Roll Forward]
Fair value beginning of period	$ (1)	$ (4)	$ (1)	$ (7)
Total gains/(losses) included in earnings	0	0	0	2
Purchases and contributions	1	4	2	9
Sales and distributions	0	(7)	(1)	(11)
Transfers:
Into Level 3	0	0	0	0
Out of Level 3	0	0	0	0
Fair value end of period	0	(7)	0	(7)
Unrealized gains/(losses) for the period included in earnings for instruments held at the end of the reporting period	0	0	0	0
Trading instruments | Trading instruments
Changes in Level 3 recurring fair value measurements, assets [Roll Forward]
Fair value beginning of period	19	3	3	1
Total gains/(losses) included in earnings	3	(1)	3	0
Purchases and contributions	22	22	53	60
Sales and distributions	(23)	(22)	(38)	(59)
Transfers:
Into Level 3	0	0	0	0
Out of Level 3	0	0	0	0
Fair value end of period	21	2	21	2
Unrealized gains/(losses) for the period included in earnings for instruments held at the end of the reporting period	5	1	5	0
Private equity and other investments | Private equity investments
Changes in Level 3 recurring fair value measurements, assets [Roll Forward]
Fair value beginning of period	62	59	63	56
Total gains/(losses) included in earnings	(32)	0	(32)	0
Purchases and contributions	0	0	0	3
Sales and distributions	0	0	(1)	0
Transfers:
Into Level 3	0	0	0	0
Out of Level 3	0	0	0	0
Fair value end of period	30	59	30	59
Unrealized gains/(losses) for the period included in earnings for instruments held at the end of the reporting period	(32)	0	(32)	0
Private equity and other investments | Other investments
Changes in Level 3 recurring fair value measurements, assets [Roll Forward]
Fair value beginning of period	24	67	24	67
Total gains/(losses) included in earnings	(2)	0	(2)	0
Purchases and contributions	0	0	0	0
Sales and distributions	0	0	0	0
Transfers:
Into Level 3	0	0	0	0
Out of Level 3	0	0	0	0
Fair value end of period	22	67	22	67
Unrealized gains/(losses) for the period included in earnings for instruments held at the end of the reporting period	$ (2)	$ 0	$ (2)	$ 0