| **Allowance for Doubtful Accounts - Impaired Finance Receivables (Details) (USD $)In Millions, unless otherwise specified**                                                                                                                                                                                                                                                                                              | 6 Months Ended |        |        |     |        |     |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|--------|--------|-----|--------|-----|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|--------|
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| **Finance Receivable, Impaired [Line Items]**                                                                                                                                                                                                                                                                                                                                                                            |           |   |   |     |   |     |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Impaired Financing Receivable, Average Recorded Investment                                                                                                                                                                                                                                                                                                                                                               | $ 13           | $ 17   |   |     |   |     |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Impaired Financing Receivable, Recorded Investment                                                                                                                                                                                                                                                                                                                                                                       |           |   | 12     | [1] | 14     | [1] | 12     | 14     | 0      | 0      | 1      | 1      | 1      | 1      | 0      | 0      |   |   |   |   |   |   |   |   |   |   |   |   |
| Specific loss reserves on impaired finance receivables                                                                                                                                                                                                                                                                                                                                                                   |           |   |   |     |   |     |   |   |   |   |   |   |   |   |   |   | 7      | 9      | 7      | 9      | 0      | 0      |   |   |   |   |   |   |
| Finance receivables on non-accrual status                                                                                                                                                                                                                                                                                                                                                                                |           |   |   |     |   |     |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   | $ 9    | $ 10   | $ 9    | $ 10 | $ 0 | $ 0 |
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| [1]Certain impaired finance receivables are measured at fair value on a nonrecurring basis. An impairment charge is recorded for the amount by which the carrying value of the receivables exceeds the fair value of the underlying collateral, net of remarketing costs. Fair values of the underlying collateral are determined by reference to dealer vehicle value publications adjusted for certain market factors. |

| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| 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 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5111-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524Reference 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=7512638&loc=d3e5258-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953550-111524
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| 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 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5212-111524
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