Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	12 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 1,197us-gaap_NetIncomeLoss	$ 857us-gaap_NetIncomeLoss	$ 1,331us-gaap_NetIncomeLoss
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on available-for-sale marketable securities, net of tax	64	(11)	64
Reclassification adjustment for net (gains) on available-for-sale marketable securities included in investment and other income, net of tax	(44)	0	(13)
Other comprehensive income (loss)	20	(11)	51
Comprehensive income	$ 1,217us-gaap_ComprehensiveIncomeNetOfTax	$ 846us-gaap_ComprehensiveIncomeNetOfTax	$ 1,382us-gaap_ComprehensiveIncomeNetOfTax

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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