| **Investments in Marketable Securities (Tables)**                                 | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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| **Investments in Marketable Securities [Abstract]**                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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| Summary of Investments in Marketable Securities                                   |     March 31, 2014    Amortized  Unrealized    Unrealized  Fair (Dollars in millions)cost  gains losses  valueAvailable-for-sale securities:            Debt instruments:             U.S. government and agency obligations$ 652 $ 1 $ (1) $ 652  Municipal debt securities  10   1   -   11  Certificates of deposit   1,599   -   -   1,599  Commercial paper  507   -   -   507  Corporate debt securities  164   6   (1)   169  Mortgage-backed securities:              U.S. government agency  60   1   (1)   60   Non-agency residential  4   1   -   5   Non-agency commercial  44   1   (2)   43  Asset-backed securities  27   -   -   27 Equity instruments:             Fixed income mutual funds:              Short-term sector fund  41   3   -   44   U.S. government sector fund  329   -   (2)   327   Municipal sector fund  21   1   -   22   Investment grade corporate sector fund  283   33   -   316   High-yield sector fund  38   7   -   45   Real return sector fund  275   -   (1)   274   Mortgage sector fund  519   1   -   520   Asset-backed securities sector fund  40   10   -   50   Emerging market sector fund  65   1   -   66   International sector fund  170   2   (1)   171  Equity mutual fund  217   264   -   481Total investments in marketable securities$ 5,065 $ 333 $ (9) $ 5,389Note 3 – Investments in Marketable Securities (Continued)                    March 31, 2013    Amortized  Unrealized    Unrealized  Fair (Dollars in millions)cost  gains losses  valueAvailable-for-sale securities:            Debt instruments:             U.S. government and agency obligations$ 101 $ 3 $ - $ 104  Municipal debt securities  14   2   -   16  Certificates of deposit   2,040   1   -   2,041  Commercial paper  495   -   -   495  Foreign government debt securities  3   -   -   3  Corporate debt securities  122   6   -   128  Mortgage-backed securities:              U.S. government agency  83   4   -   87   Non-agency residential  4   1   -   5   Non-agency commercial  50   1   -   51  Asset-backed securities  13   -   -   13 Equity instruments:             Fixed income mutual funds:              Short-term sector fund  40   3   -   43   U.S. government sector fund  296   16   -   312   Municipal sector fund  19   3   -   22   Investment grade corporate sector fund  273   54   -   327   High-yield sector fund  34   8   -   42   Real return sector fund  284   9   -   293   Mortgage sector fund  663   -   (15)   648   Asset-backed securities sector fund  38   9   -   47   Emerging market sector fund  63   3   -   66   International sector fund  163   7   -   170  Equity mutual fund  259   225   -   484Total investments in marketable securities$ 5,057 $ 355 $ (15) $ 5,397 |
| Summary of the aging of fair value and gross unrealized losses for AFS securities |       Less than 12 months as of      March 31, 2014 March 31, 2013      Fair  Unrealized  Fair  Unrealized(Dollars in millions) value   losses value   losses Available-for-sale securities:             Debt instruments:               U.S. government and agency                obligations $ 33 $ (1) $ - $ -   Corporate debt securities   43   (1)   -   -   Mortgage backed securities:                U.S. government agency   35   (1)   -   - Equity instruments:              Fixed income mutual funds:               U.S. government sector fund   270   (2)   -   -   Real return sector fund   268   (1)   -   -   Mortgage sector fund   -   -   532   (12)   International sector fund   138   (1)   -   -Total investments in marketable    securities $ 787 $ (7) $ 532 $ (12)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Schedule of realized gains and losses on sales from AFS                           |    Years Ended March 31,(Dollars in millions) 2014 2013 2012Available-for-sale securities:       Realized gains on sales$ 59$ 23$ 16 Realized losses on sales$ (4)$ (2)$ (41) Other-than-temporary impairment $ (55)$ -$ -                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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| Summary of Contractual Maturities of Available-for-Sale Securities                |   Due in 1 Year or Due after 1 Year Due after 5 Years            Lessthrough 5 Yearsthrough 10 YearsDue after 10 YearsTotal(Dollars in millions)Amount YieldAmount YieldAmount YieldAmount YieldAmount YieldFair Value of Available-for-Sale Securities:                    Debt instruments:                         U.S. government and                           agency obligations$ 570 0.13%$ 59 1.57%$ 23 1.61%$ - 0.00%$ 652 0.81%Municipal debt securities  - 0.00   - 0.00   2 5.58   9 5.57   11 5.58 Certificates of deposit  1,599 0.23   - 0.00   - 0.00   - 0.00   1,599 0.23 Commercial paper  507 0.17   - 0.00   - 0.00   - 0.00   507 0.17 Corporate debt                           securities  2 5.51   63 3.67   64 3.75   40 4.68   169 3.90 Mortgage-backed securities:                          U.S. government agency   - 0.00   1 5.78   3 3.45   56 3.45   60 3.47  Non-agency residential  - 0.00   - 0.00   - 0.00   5 9.49   5 9.49  Non-agency commercial  - 0.00  3 2.24   1 3.28   39 3.11   43 3.06 Asset-backed securities  - 0.00   5 0.75   4 1.88   18 2.66   27 2.08 Debt instruments total  2,678 0.20   131 2.59   97 3.20   167 3.87   3,073 0.70                          Equity instruments:                        Fixed income mutual funds                      1,835 4.41 Equity mutual funds                      481 4.27 Equity instruments total  -     -     -     -     2,316 4.38                           Total fair value$ 2,678 0.20%$ 131 2.59%$ 97 3.20%$ 167 3.87%$ 5,389 2.28%Total amortized cost$ 2,679   $ 127   $ 97   $ 164   $ 5,065                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |

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| - Definition
Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.
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Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
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| - Definition
Tabular disclosure of the proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales.
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| - Definition
For all investments in an unrealized loss position, including those for which other-than-temporary impairments have not been recognized in earnings (including investments for which a portion of an other-than-temporary impairment has been recognized in other comprehensive income), a tabular disclosure of the aggregate related fair value of investments with unrealized losses and the aggregate amount of unrealized losses (that is, the amount by which amortized cost basis exceeds fair value).
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