Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 1,331	$ 1,486	$ 1,853
Other comprehensive income, net of tax:
Net unrealized gains on available-for-sale marketable securities, net of tax	64	43	4
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in net income, net of tax	(13)	17	(8)
Other comprehensive income (loss)	51	60	(4)
Comprehensive income	$ 1,382	$ 1,546	$ 1,849