| **Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2014 | Mar. 31, 2013 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Other assets:**                                                                                                                                                     |          |          |
| Carrying value of derivative contracts - Other assets                                                                                                                 | $ 49          | $ 58          |
| Collateral held                                                                                                                                                       | 718           | 953           |
| Collateral posted in excess of the fair value of derivative liabilities                                                                                               | 3             | 6             |
| **Other liabilities:**                                                                                                                                                |          |          |
| Carrying value of derivative contracts - Other liabilities                                                                                                            | 6             | 17            |
| Collateral posted                                                                                                                                                     | 331           | 184           |
| Collateral held in excess of the fair value of derivative assets                                                                                                      | 5             | 3             |
| Other assets [Member]                                                                                                                                                 |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 34,633        | 31,545        |
| Fair value                                                                                                                                                            | 1,235         | 1,719         |
| Counterparty netting and collateral                                                                                                                                   | (1,186)       | (1,661)       |
| Carrying value of derivative contracts - Other assets                                                                                                                 | 49            | 58            |
| Other assets [Member] | Interest rate swap [Member]                                                                                                                   |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 26,407        | 22,801        |
| Fair value                                                                                                                                                            | 361           | 580           |
| Other assets [Member] | Foreign currency swaps [Member]                                                                                                               |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 8,226         | 8,744         |
| Fair value                                                                                                                                                            | 874           | 1,139         |
| Other liabilities [Member]                                                                                                                                            |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 61,718        | 55,349        |
| Fair value                                                                                                                                                            | 805           | 909           |
| Counterparty netting and collateral                                                                                                                                   | (799)         | (892)         |
| Carrying value of derivative contracts - Other liabilities                                                                                                            | 6             | 17            |
| Other liabilities [Member] | Interest rate swap [Member]                                                                                                              |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 57,689        | 51,342        |
| Fair value                                                                                                                                                            | 553           | 766           |
| Other liabilities [Member] | Interest rate cap [Member]                                                                                                               |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 50            | 50            |
| Fair value                                                                                                                                                            | 0             | 0             |
| Other liabilities [Member] | Foreign currency swaps [Member]                                                                                                          |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 3,979         | 3,893         |
| Fair value                                                                                                                                                            | 252           | 131           |
| Other liabilities [Member] | Embedded derivatives                                                                                                                     |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              |          | 64            |
| Fair value                                                                                                                                                            |          | 12            |
| Hedge accounting derivatives [Member] | Other assets [Member]                                                                                                         |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 1,317         | 1,711         |
| Fair value                                                                                                                                                            | 367           | 535           |
| Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]                                                                           |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 465           | 465           |
| Fair value                                                                                                                                                            | 25            | 44            |
| Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]                                                                       |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 852           | 1,246         |
| Fair value                                                                                                                                                            | 342           | 491           |
| Hedge accounting derivatives [Member] | Other liabilities [Member]                                                                                                    |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 157           | 790           |
| Fair value                                                                                                                                                            | 14            | 29            |
| Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]                                                                      |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 0             | 0             |
| Fair value                                                                                                                                                            | 0             | 0             |
| Hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [Member]                                                                       |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 0             | 0             |
| Fair value                                                                                                                                                            | 0             | 0             |
| Hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]                                                                  |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 157           | 790           |
| Fair value                                                                                                                                                            | 14            | 29            |
| Hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives                                                                             |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              |          | 0             |
| Fair value                                                                                                                                                            |          | 0             |
| Non-hedge accounting derivatives [Member] | Other assets [Member]                                                                                                     |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 33,316        | 29,834        |
| Fair value                                                                                                                                                            | 868           | 1,184         |
| Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]                                                                       |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 25,942        | 22,336        |
| Fair value                                                                                                                                                            | 336           | 536           |
| Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]                                                                   |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 7,374         | 7,498         |
| Fair value                                                                                                                                                            | 532           | 648           |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member]                                                                                                |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 61,561        | 54,559        |
| Fair value                                                                                                                                                            | 791           | 880           |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]                                                                  |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 57,689        | 51,342        |
| Fair value                                                                                                                                                            | 553           | 766           |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate cap [Member]                                                                   |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 50            | 50            |
| Fair value                                                                                                                                                            | 0             | 0             |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Foreign currency swaps [Member]                                                              |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 3,822         | 3,103         |
| Fair value                                                                                                                                                            | 238           | 102           |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives                                                                         |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              |          | 64            |
| Fair value                                                                                                                                                            |          | $ 12          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impact of offsetting the fair value amounts of derivative assets and liabilities executed with the same counterparty under the same master netting agreement. This also includes the amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions. Positive values represent increases.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impact of offsetting the fair value amounts of derivative assets and liabilities executed with the same counterparty under the same master netting agreement. This also includes the amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions. Positive values represent increases.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying value of derivative assets presented in other assets of the balance sheet.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of collateral held by a party to a derivative contract (Party A), which exceeds the fair value of the derivative asset of Party A with its counterparty (Party B).
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of collateral posted by a party to a derivative contract (Party A), which exceeds the fair value of the derivative liability of Party A, to its counterparty (Party B).
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying value of derivative contracts disclosed in Other Liabilities on the balance sheet.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Nominal or face amount used to calculate payments on the derivative asset.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.
+ References
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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Nominal or face amount used to calculate payments on the derivative liability.
+ References
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