Fair Value Measurements (Schedule of the Carrying Value of Assets Measured at Fair Value on a Recurring Basis) (Details) - Recurring - USD ($) $ in Millions | Jun. 30, 2017 | Mar. 31, 2017 |
Fair Value | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | $ 2,314 | $ 4,256 |
Fair Value | Cash | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 480 | 1,183 |
Fair Value | Non-negotiable certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 39 | 15 |
Fair Value | Level 1 (Quoted prices in active markets for identical assets): | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 1,408 | 2,635 |
Fair Value | Level 1 (Quoted prices in active markets for identical assets): | Money market | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 1,298 | 2,532 |
Fair Value | Level 1 (Quoted prices in active markets for identical assets): | U.S. government securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 102 | 94 |
Fair Value | Level 1 (Quoted prices in active markets for identical assets): | Marketable equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 8 | 9 |
Fair Value | Level 2 (Significant other observable inputs): | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 387 | 423 |
Fair Value | Level 2 (Significant other observable inputs): | U.S. agency securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 64 | 75 |
Fair Value | Level 2 (Significant other observable inputs): | Commercial paper | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 323 | 348 |
Reported Value Measurement | Cash and Cash Equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 2,306 | 4,247 |
Reported Value Measurement | Cash and Cash Equivalents | Cash | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 480 | 1,183 |
Reported Value Measurement | Cash and Cash Equivalents | Non-negotiable certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 39 | 15 |
Reported Value Measurement | Cash and Cash Equivalents | Level 1 (Quoted prices in active markets for identical assets): | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 1,400 | 2,626 |
Reported Value Measurement | Cash and Cash Equivalents | Level 1 (Quoted prices in active markets for identical assets): | Money market | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 1,298 | 2,532 |
Reported Value Measurement | Cash and Cash Equivalents | Level 1 (Quoted prices in active markets for identical assets): | U.S. government securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 102 | 94 |
Reported Value Measurement | Cash and Cash Equivalents | Level 1 (Quoted prices in active markets for identical assets): | Marketable equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Cash and Cash Equivalents | Level 2 (Significant other observable inputs): | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 387 | 423 |
Reported Value Measurement | Cash and Cash Equivalents | Level 2 (Significant other observable inputs): | U.S. agency securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 64 | 75 |
Reported Value Measurement | Cash and Cash Equivalents | Level 2 (Significant other observable inputs): | Commercial paper | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 323 | 348 |
Reported Value Measurement | Short-Term Investments | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 8 | 9 |
Reported Value Measurement | Short-Term Investments | Cash | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Short-Term Investments | Non-negotiable certificates of deposit | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Short-Term Investments | Level 1 (Quoted prices in active markets for identical assets): | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 8 | 9 |
Reported Value Measurement | Short-Term Investments | Level 1 (Quoted prices in active markets for identical assets): | Money market | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Short-Term Investments | Level 1 (Quoted prices in active markets for identical assets): | U.S. government securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Short-Term Investments | Level 1 (Quoted prices in active markets for identical assets): | Marketable equity securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 8 | 9 |
Reported Value Measurement | Short-Term Investments | Level 2 (Significant other observable inputs): | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Short-Term Investments | Level 2 (Significant other observable inputs): | U.S. agency securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | 0 | 0 |
Reported Value Measurement | Short-Term Investments | Level 2 (Significant other observable inputs): | Commercial paper | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Assets, fair value disclosure | $ 0 | $ 0 |