BORROWINGS (SHUSA) (Narrative) (Details) - USD ($)	Sep. 30, 2020	Sep. 30, 2019
Short-term debt borrowings due February 2021
Debt Instrument [Line Items]
Debt issued	$ 125,000,000.0
Stated interest rate	2.00%
Senior Notes
Debt Instrument [Line Items]
Debt issued	$ 2,100,000,000	$ 2,500,000,000
Senior Notes | 5.83% senior notes, due March 2023
Debt Instrument [Line Items]
Debt issued	$ 500,000,000.0
Stated interest rate	5.83%
Senior Notes | Senior fixed rate note due April 2023
Debt Instrument [Line Items]
Debt issued	$ 447,100,000
Senior Notes | 3.45% senior fixed rate note due June 2025
Debt Instrument [Line Items]
Debt issued	$ 1,000,000,000.0
Stated interest rate	3.45%
Senior Notes | 2.65% senior notes due April 2020
Debt Instrument [Line Items]
Stated interest rate	2.65%
Debt repurchased	$ 1,000,000,000.0
Senior Notes | Senior floating rate note due September 2020
Debt Instrument [Line Items]
Debt repurchased	$ 114,500,000
Senior Notes | 3.50% senior notes due 2024
Debt Instrument [Line Items]
Debt issued		$ 1,000,000,000.0
Stated interest rate		3.50%
Senior Notes | Senior floating rate notes due 2022
Debt Instrument [Line Items]
Debt issued		$ 720,900,000
Senior Notes | Senior fixed rate notes due 2024
Debt Instrument [Line Items]
Debt issued		$ 750,000,000.0
Stated interest rate		2.88%
Senior Notes | 2.70% senior notes due May 2019
Debt Instrument [Line Items]
Stated interest rate		2.70%
Debt repurchased		$ 178,700,000
Senior Notes | Senior floating rate notes due July 2019
Debt Instrument [Line Items]
Debt repurchased		388,700,000
Senior Notes | Senior floating rate notes due September 2019
Debt Instrument [Line Items]
Debt repurchased		$ 371,000,000.0