FINANCIAL SERVICES - Aging Customer Financing Receivables, Gross, Including Accrued Interest (Details) - USD ($) $ in Millions	Jul. 31, 2020	Jan. 31, 2020
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross	$ 10,033
Customer receivables
Financing Receivable, Past Due [Line Items]
Current	9,524	$ 8,739
Total customer receivables, gross	10,033	9,310
Revolving
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross		9,310
Revolving | Customer receivables
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross	778	824
Revolving | Revolving — DPA
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross		621
Revolving | Revolving — DPA | Customer receivables
Financing Receivable, Past Due [Line Items]
Current	556	550
Total customer receivables, gross	598	621
Revolving | Revolving — DBC
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross		203
Revolving | Revolving — DBC | Customer receivables
Financing Receivable, Past Due [Line Items]
Current	165	184
Total customer receivables, gross	180	203
Fixed-term | Customer receivables
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross	9,255	8,486
Fixed-term | Fixed-term — Consumer and Commercial
Financing Receivable, Past Due [Line Items]
Total customer receivables, gross		8,486
Fixed-term | Fixed-term — Consumer and Commercial | Customer receivables
Financing Receivable, Past Due [Line Items]
Current	8,803	8,005
Total customer receivables, gross	9,255	8,486
Past Due 1 — 90 Days | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	421	439
Past Due 1 — 90 Days | Revolving | Revolving — DPA | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	29	51
Past Due 1 — 90 Days | Revolving | Revolving — DBC | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	11	15
Past Due 1 — 90 Days | Fixed-term | Fixed-term — Consumer and Commercial | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	381	373
Past Due >90 Days | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	88	132
Past Due >90 Days | Revolving | Revolving — DPA | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	13	20
Past Due >90 Days | Revolving | Revolving — DBC | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	4	4
Past Due >90 Days | Fixed-term | Fixed-term — Consumer and Commercial | Customer receivables
Financing Receivable, Past Due [Line Items]
Past due	$ 71	$ 108