| **Allowance for doubtful accounts (Impaired Finance Receivables) (Details) (USD $)In Millions, unless otherwise specified** | 6 Months Ended |        |
|-----------------------------------------------------------------------------------------------------------------------------|----------------|--------|
||
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Specific loss reserves on impaired finance receivables                                                                      | $ 10           |   | $ 10 |
| Finance receivables on non-accrual status                                                                                   | 12             |   | 14 |
| Impaired financing receivable without specific loss reserves [Member]                                                       |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                          | 1              |   | 2 |
| Impaired finance receivables with specific loss reserves [Member]                                                           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                          | 16             |   | 15 |
| Retail Portfolio [Member]                                                                                                   |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Specific loss reserves on impaired finance receivables                                                                      | 10             |   | 10 |
| Finance receivables on non-accrual status                                                                                   | 12             |   | 14 |
| Average balance of impaired finance receivables                                                                             | 17             | 39     |   |
| Retail Portfolio [Member] | Impaired financing receivable without specific loss reserves [Member]                           |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                          | 1              |   | 2 |
| Retail Portfolio [Member] | Impaired finance receivables with specific loss reserves [Member]                               |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                          | 16             |   | 15 |
| Wholesale Portfolio [Member]                                                                                                |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Specific loss reserves on impaired finance receivables                                                                      | 0              |   | 0 |
| Finance receivables on non-accrual status                                                                                   | 0              |   | 0 |
| Wholesale Portfolio [Member] | Impaired financing receivable without specific loss reserves [Member]                        |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                          | 0              |   | 0 |
| Wholesale Portfolio [Member] | Impaired finance receivables with specific loss reserves [Member]                            |           |   |   |
| **Financing Receivable, Impaired [Line Items]**                                                                             |           |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                          | $ 0            |   | $ 0 |

| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableImpairedLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5111-111524 -Subparagraph (d)
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The average recorded investment related to impaired financing receivables.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5212-111524 -Subparagraph (c)(1)Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5258-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The recorded investment related to impaired financing receivables.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=6965416&loc=SL6953550-111524 -Subparagraph (b)
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of allowance for credit losses related to recorded investment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=6965416&loc=d3e5212-111524 -Subparagraph (a)(3)
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRelatedAllowance**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |