BANK DEPOSITS, Summary of Interest Expense on Deposits (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Interest expense on deposits [Abstract]
Savings, money market, and NOW accounts	$ 6	$ 34	$ 18	$ 67
Certificates of deposit	6	3	10	5
Total interest expense on deposits	$ 12	$ 37	$ 28	$ 72

Aggregate amount of interest expense on all deposits.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.6)
-URI http://asc.fasb.org/extlink&oid=116637391&loc=SL114874048-224260

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- References

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- Definition

The aggregate interest expense incurred on the following for deposits in domestic offices: (1) transaction accounts (NOW, and telephone and preauthorized transfer accounts), (2) money market deposits and (3) savings deposits.

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- Definition

The aggregate interest expense incurred on time deposits, including certificates of deposits, in domestic offices.

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