| **Financing Arrangement**       | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               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| **Debt Instruments [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              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| Financing Arrangement           | (10) FINANCING ARRANGEMENTS0.75% Convertible Senior Notes Due 2016In July 2011, we issued $632.5 million aggregate principal amount of 0.75% Convertible Senior Notes due 2016 (the “Convertible Notes”). The Convertible Notes matured on July 15, 2016. The Convertible Notes were senior unsecured obligations which paid interest semiannually in arrears at a rate of 0.75% per annum on January 15 and July 15 of each year. The Convertible Notes were convertible into cash and shares of our common stock based on an initial conversion value of 31.5075 shares of our common stock per $1,000 principal amount of Convertible Notes (equivalent to an initial conversion price of approximately $31.74 per share). Upon conversion of the Convertible Notes, holders received cash up to the principal amount of each Convertible Note, and any excess conversion value was delivered in shares of our common stock. The Convertible Notes do not contain any financial covenants.Upon conversion of any Convertible Notes, we delivered cash up to the principal amount of the Convertible Notes and any excess conversion value was delivered in shares of our common stock. During the three months ended September 30, 2016, approximately $136 million principal value of the Convertible Notes were converted by holders thereof. During the three months ended September 30, 2016, we repaid the remaining principal balance on the Convertible Notes of $136 million and issued approximately 2.5 million shares of common stock to noteholders with a fair value of $193 million on the final maturity date of July 15, 2016. We also received and cancelled approximately 2.5 million shares of common stock from the exercise of the Convertible Note Hedge during the three months ended September 30, 2016. During the six months ended September 30, 2016, we repaid $163 million of the principal balance of the Convertible Notes and issued approximately 2.9 million shares of common stock to noteholders with a fair value of $222 million, resulting in a loss on extinguishment of $0.3 million.The carrying and fair values of the Convertible Notes are as follows (in millions):   As of September 30, 2016 As of March 31, 2016Principal amount of Convertible Notes$— $163Unamortized debt discount of the liability component— (2)Net carrying value of Convertible Notes$— $161    Fair value of Convertible Notes (Level 2)$— $338Convertible Note Hedge and Warrants IssuanceIn July 2011, we entered into certain agreements designed to reduce the potential dilution with respect to our common stock upon conversion of the Convertible Notes (“the Convertible Note Hedge”). We paid $107 million for the Convertible Note Hedge, which was recorded as an equity transaction. The Convertible Note Hedge, subject to customary anti-dilution adjustments, provided us with the option to acquire, on a net settlement basis, approximately 19.9 million shares of our common stock, equal to the number of shares of our common stock that notionally underlie the Convertible Notes at a strike price of $31.74, which corresponds to the conversion price of the Convertible Notes. During the three and six months ended September 30, 2016, we received 2.5 million and 2.9 million shares, respectively, of our common stock under the Convertible Note Hedge. Separately, in July 2011 we also entered into privately negotiated warrant transactions with certain counterparties whereby we sold to independent third parties warrants (the “Warrants”) to acquire, subject to customary anti-dilution adjustments that are substantially the same as the anti-dilution provisions contained in the Convertible Notes, up to 19.9 million shares of our common stock (which is also equal to the number of shares of our common stock that notionally underlie the Convertible Notes), with a strike price of $41.14. The Warrants have a dilutive effect with respect to our common stock to the extent that the market price per share of our common stock exceeds $41.14 on or prior to the expiration date of the Warrants. The Warrants automatically exercise over a 60 trading day period that began on October 17, 2016. Based on the closing price of our common stock of$85.40at the end of the quarter endedSeptember 30, 2016, approximately10.3 millionshares of our common stock would be issuable to Warrants holders. The actual amount of shares issuable upon exercise will be determined based upon the market price of our common stock during the 60 day trading period that began on October 17, 2016. We received proceeds of $65 million from the sale of the Warrants in fiscal year 2012.Effect of conversion on earning per share (“EPS”)Prior to conversion of the Convertible Notes, we included the effect of the additional potential dilutive shares if our common stock price exceeded $31.74 per share using the treasury stock method. If the average price of our common stock exceeds $41.14 per share for a quarterly period, we also include the effect of the additional potential dilutive shares related to the Warrants using the treasury stock method. Prior to conversion, the Convertible Note Hedge was not considered for purposes of the EPS calculation, as its effect would have been anti-dilutive. Upon conversion, the Convertible Note Hedge offset the dilutive effect of the Notes when the stock price was above $31.74 per share. See Note 13 for additional information related to our EPS.Senior NotesIn February 2016, we issued $600 million aggregate principal amount of 3.70% Senior Notes due March 1, 2021 (the “2021 Notes”) and $400 million aggregate principal amount of 4.80% Senior Notes due March 1, 2026 (the “2026 Notes,” and together with the 2021 Notes, the “Senior Notes”). Our proceeds were $989 million, net of discount of $2 million and issuance costs of $9 million. Both the discount and issuance costs are being amortized to interest expense over the respective terms of the 2021 Notes and the 2026 Notes using the effective interest rate method. The effective interest rate was 3.94% for the 2021 Notes and 4.97% for the 2026 Notes. Interest is payable semiannually in arrears, on March 1 and September 1 of each year.The carrying and fair values of the Senior Notes are as follows (in millions):   As ofSeptember 30, 2016 As ofMarch 31, 2016Senior Notes:   3.70% Senior Notes due 2021$600 $6004.80% Senior Notes due 2026400 400Total principal amount$1,000 $1,000Unaccreted discount(2) (2)Unamortized debt issuance costs(8) (9)Net carrying value of Senior Notes$990 $989    Fair value of Senior Notes (Level 2)$1,082 $1,039As of September 30, 2016, the remaining life of the 2021 Notes and 2026 Notes is approximately 4.4 and 9.4 years, respectively.The Senior Notes are senior unsecured obligations and rank equally with all our other existing and future unsubordinated obligations and any indebtedness that we may incur from time to time under our Credit Facility. The 2021 Notes and the 2026 Notes are redeemable at our option at any time prior to February 1, 2021 or December 1, 2025, respectively, subject to a make-whole premium. Within one and three months of maturity, we may redeem the 2021 Notes or the 2026 Notes, respectively, at a redemption price equal to 100% of the aggregate principal amount plus accrued and unpaid interest. In addition, upon the occurrence of a change of control repurchase event, the holders of the Senior Notes may require us to repurchase all or a portion of the Senior Notes, at a price equal to 101% of their principal amount, plus accrued and unpaid interest to the date of repurchase. The Senior Notes also include covenants that limit our ability to incur liens on assets and to enter into sale and leaseback transactions, subject to certain allowances.Credit FacilityIn March 2015, we entered into a $500 million senior unsecured revolving credit facility (“Credit Facility”) with a syndicate of banks. The Credit Facility terminates on March 19, 2020. The Credit Facility contains an option to arrange with existing lenders and/or new lenders to provide up to an aggregate of $250 million in additional commitments for revolving loans. Proceeds of loans made under the Credit Facility may be used for general corporate purposes.The loans bear interest, at our option, at the base rate plus an applicable spread or an adjusted LIBOR rate plus an applicable spread, in each case with such spread being determined based on our consolidated leverage ratio for the preceding fiscal quarter. We are also obligated to pay other customary fees for a credit facility of this size and type. Interest is due and payable in arrears quarterly for loans bearing interest at the base rate and at the end of an interest period (or at each three month interval in the case of loans with interest periods greater than three months) in the case of loans bearing interest at the adjusted LIBOR rate. Principal, together with all accrued and unpaid interest, is due and payable on March 19, 2020. The credit agreement contains customary affirmative and negative covenants, including covenants that limit or restrict our ability to, among other things, incur subsidiary indebtedness, grant liens, dispose of all or substantially all assets and pay dividends or make distributions, in each case subject to customary exceptions for a credit facility of this size and type. We are also required to maintain compliance with a capitalization ratio and maintain a minimum level of total liquidity.The credit agreement contains customary events of default, including among others, non-payment defaults, covenant defaults, cross-defaults to material indebtedness, bankruptcy and insolvency defaults, material judgment defaults and a change of control default, in each case, subject to customary exceptions for a credit facility of this size and type. The occurrence of an event of default could result in the acceleration of the obligations under the credit facility, an obligation by any guarantors to repay the obligations in full and an increase in the applicable interest rate.As of September 30, 2016, no amounts were outstanding under the Credit Facility. $2 million of debt issuance costs that were paid in connection with obtaining this credit facility are being amortized to interest expense over the 5-year term of the Credit Facility.   Interest ExpenseThe following table summarizes our interest expense recognized for the three and six months ended September 30, 2016 and 2015 that is included in interest and other income (expense), net on our Condensed Consolidated Statements of Operations (in millions):  Three Months Ended September 30, Six Months Ended September 30, 2016 2015 2016 2015Amortization of debt discount(1) (5) (2) (11)Amortization of debt issuance costs— — (1) (1)Coupon interest expense(10) (1) (21) (2)Total interest expense$(11) $(6) $(24) $(14) |

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| - References
No definition available.
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| - Definition
The entire disclosure for long-term debt.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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