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