| **Derivatives, Hedging Activities and Interest Expense (Effect of Derivative Instruments on Balance Sheet) (Details) (USD $)In Millions, unless otherwise specified** | Dec. 31, 2013 | Mar. 31, 2013 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Other assets:**                                                                                                                                                     |          |          |
| Carrying value of derivative contracts - Other assets                                                                                                                 | $ 49          | $ 58          |
| Collateral held                                                                                                                                                       | 794           | 953           |
| Collateral posted in excess of the fair value of derivative liabilities                                                                                               | 9             | 6             |
| **Other liabilities:**                                                                                                                                                |          |          |
| Carrying value of derivative contracts - Other liabilities                                                                                                            | 13            | 17            |
| Collateral posted                                                                                                                                                     | 476           | 184           |
| Collateral held in excess of the fair value of derivative assets                                                                                                      | 32            | 3             |
| Other assets [Member]                                                                                                                                                 |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 33,041        | 31,545        |
| Fair value                                                                                                                                                            | 1,372         | 1,719         |
| Counterparty netting and collateral                                                                                                                                   | (1,323)       | (1,661)       |
| Carrying value of derivative contracts - Other assets                                                                                                                 | 49            | 58            |
| Other assets [Member] | Interest rate swap [Member]                                                                                                                   |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 24,867        | 22,801        |
| Fair value                                                                                                                                                            | 378           | 580           |
| Other assets [Member] | Foreign currency swaps [Member]                                                                                                               |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 8,174         | 8,744         |
| Fair value                                                                                                                                                            | 994           | 1,139         |
| Other assets [Member] | Embedded derivatives                                                                                                                          |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 0             | 0             |
| Fair value                                                                                                                                                            | 0             | 0             |
| Other liabilities [Member]                                                                                                                                            |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 59,338        | 55,349        |
| Fair value                                                                                                                                                            | 1,018         | 909           |
| Counterparty netting and collateral                                                                                                                                   | (1,005)       | (892)         |
| Carrying value of derivative contracts - Other liabilities                                                                                                            | 13            | 17            |
| Other liabilities [Member] | Interest rate swap [Member]                                                                                                              |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 55,265        | 51,342        |
| Fair value                                                                                                                                                            | 656           | 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                                                                                                                                                              | 4,023         | 3,893         |
| Fair value                                                                                                                                                            | 362           | 131           |
| Other liabilities [Member] | Embedded derivatives                                                                                                                     |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 0             | 64            |
| Fair value                                                                                                                                                            | 0             | 12            |
| Hedge accounting derivatives [Member] | Other assets [Member]                                                                                                         |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 1,647         | 1,711         |
| Fair value                                                                                                                                                            | 501           | 535           |
| Hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]                                                                           |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 465           | 465           |
| Fair value                                                                                                                                                            | 31            | 44            |
| Hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]                                                                       |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 1,182         | 1,246         |
| Fair value                                                                                                                                                            | 470           | 491           |
| Hedge accounting derivatives [Member] | Other assets [Member] | Embedded derivatives                                                                                  |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 0             | 0             |
| Fair value                                                                                                                                                            | 0             | 0             |
| Hedge accounting derivatives [Member] | Other liabilities [Member]                                                                                                    |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 157           | 790           |
| Fair value                                                                                                                                                            | 10            | 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                                                                                                                                                            | 10            | 29            |
| Hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives                                                                             |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 0             | 0             |
| Fair value                                                                                                                                                            | 0             | 0             |
| Non-hedge accounting derivatives [Member] | Other assets [Member]                                                                                                     |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 31,394        | 29,834        |
| Fair value                                                                                                                                                            | 871           | 1,184         |
| Non-hedge accounting derivatives [Member] | Other assets [Member] | Interest rate swap [Member]                                                                       |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 24,402        | 22,336        |
| Fair value                                                                                                                                                            | 347           | 536           |
| Non-hedge accounting derivatives [Member] | Other assets [Member] | Foreign currency swaps [Member]                                                                   |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 6,992         | 7,498         |
| Fair value                                                                                                                                                            | 524           | 648           |
| Non-hedge accounting derivatives [Member] | Other assets [Member] | Embedded derivatives                                                                              |          |          |
| **Other assets:**                                                                                                                                                     |          |          |
| Notional                                                                                                                                                              | 0             | 0             |
| Fair value                                                                                                                                                            | 0             | 0             |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member]                                                                                                |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 59,181        | 54,559        |
| Fair value                                                                                                                                                            | 1,008         | 880           |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Interest rate swap [Member]                                                                  |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 55,265        | 51,342        |
| Fair value                                                                                                                                                            | 656           | 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                                                                                                                                                              | 3,866         | 3,103         |
| Fair value                                                                                                                                                            | 352           | 102           |
| Non-hedge accounting derivatives [Member] | Other liabilities [Member] | Embedded derivatives                                                                         |          |          |
| **Other liabilities:**                                                                                                                                                |          |          |
| Notional                                                                                                                                                              | 0             | 64            |
| Fair value                                                                                                                                                            | $ 0           | $ 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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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 asset 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
Carrying value of derivative liability presented in other liabilities of the balance sheet.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total monetary amount specified by the derivative asset(s). Expressed as an absolute value.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the obligation to return cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the right to reclaim cash collateral under master netting arrangements that have not been offset against net derivative instrument positions.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=d3e41678-113959
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total monetary amount specified by the derivative liability (or liabilities). Expressed as an absolute value.
+ References
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