ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS), Schedule of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Oct. 01, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period			$ 6,643
OCI:
Total other comprehensive income/(loss), net of tax	$ (6)	$ 14	12	$ 6
Balance end of period	6,834	6,435	6,834	6,435
Net investment hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period	97	125	110	88
Cumulative effect of adoption of accounting standard					$ 0
OCI:
OCI before reclassifications and taxes	69	(15)	52	34
Amounts reclassified from AOCI, before tax	0	0	0	0
Pre-tax net OCI	69	(15)	52	34
Income tax effect	(17)	4	(13)	(8)
Total other comprehensive income/(loss), net of tax	52	(11)	39	26
Balance end of period	149	114	149	114
Currency translations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period	(113)	(161)	(135)	(111)
Cumulative effect of adoption of accounting standard					0
OCI:
OCI before reclassifications and taxes	(78)	19	(56)	(31)
Amounts reclassified from AOCI, before tax	0	0	0	0
Pre-tax net OCI	(78)	19	(56)	(31)
Income tax effect	0	0	0	0
Total other comprehensive income/(loss), net of tax	(78)	19	(56)	(31)
Balance end of period	(191)	(142)	(191)	(142)
Subtotal: net investment hedges and currency translations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period	(16)	(36)	(25)	(23)
Cumulative effect of adoption of accounting standard					0
OCI:
OCI before reclassifications and taxes	(9)	4	(4)	3
Amounts reclassified from AOCI, before tax	0	0	0	0
Pre-tax net OCI	(9)	4	(4)	3
Income tax effect	(17)	4	(13)	(8)
Total other comprehensive income/(loss), net of tax	(26)	8	(17)	(5)
Balance end of period	(42)	(28)	(42)	(28)
AOCI, Accumulated Gain (Loss), Available-for-sale, Parent
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period				(46)
Cumulative effect of adoption of accounting standard					(4)
OCI:
OCI before reclassifications and taxes				58
Amounts reclassified from AOCI, before tax				0
Income tax effect				(17)
Available- for-sale securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period	20	(28)	21
OCI:
OCI before reclassifications and taxes	85	26	83
Amounts reclassified from AOCI, before tax	0	0	0
Pre-tax net OCI	85	26	83	58
Income tax effect	(22)	(7)	(21)
Total other comprehensive income/(loss), net of tax	63	19	62	41
Balance end of period	83	(9)	83	(9)
Cash flow hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period	(9)	25	(19)	42
Cumulative effect of adoption of accounting standard					0
OCI:
OCI before reclassifications and taxes	(58)	(16)	(44)	(40)
Amounts reclassified from AOCI, before tax	0	(2)	0	(3)
Pre-tax net OCI	(58)	(18)	(44)	(43)
Income tax effect	15	5	11	13
Total other comprehensive income/(loss), net of tax	(43)	(13)	(33)	(30)
Balance end of period	(52)	12	(52)	12
Accumulated other comprehensive loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance beginning of period	(5)	(39)	(23)	(27)
Cumulative effect of adoption of accounting standard					$ (4)
OCI:
OCI before reclassifications and taxes	18	14	35	21
Amounts reclassified from AOCI, before tax	0	(2)	0	(3)
Pre-tax net OCI	18	12	35	18
Income tax effect	(24)	2	(23)	(12)
Total other comprehensive income/(loss), net of tax	(6)	14	12	6
Balance end of period	$ (11)	$ (25)	$ (11)	$ (25)