| **Fair Value Measurements - Level 3 Reconciliation (Details) (USD $)In Millions, unless otherwise specified**       | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended |        |
|---------------------------------------------------------------------------------------------------------------------|----------------|----------------|----------------|----------------|--------|
||
| **Fair Value Assets And Liablities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]**     |           |           |           |           |   |   |   |   |   |
| Other Assets                                                                                                        |           |           |           |           |   |   |   |   | $ 18 |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Beginning Value | (6)            | (7)            | (6)            | (7)            | 0      | 0 | 0 | 0 |   |
| Transfers out of Level 3                                                                                            | 0              | 0              | 0              | 0              | 0      | 0 | 0 | 0 |   |
| Issuances                                                                                                           | 0              | 0              | 0              | 0              | 0      | 0 | 0 | 0 |   |
| Settlements                                                                                                         | 0              | 0              | 0              | 0              | 0      | 0 | 0 | 0 |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Ending Value    | (6)            | (7)            | (6)            | (7)            | 0      | 0 | 0 | 0 |   |
| Change in unrealized gains on assets and liabilities still held                                                     | $ 0            | $ 0            | $ 0            | $ 0            | $ 0    | $ 0 | $ 0 | $ 0 |   |

| X                                                                                                                                                                                                                        |
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| X                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Change in Unrealized Gains on Assets and Liabilities Still Held1
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability) Ending Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Issuances
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate carrying amounts, as of the balance sheet date, of assets not separately disclosed in the balance sheet.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 17 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 10 -Article 7
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