Consolidated Statement of Comprehensive Income (USD $)	12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 857,000,000	$ 1,331,000,000	$ 1,486,000,000
Other comprehensive income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities, net of tax	(11,000,000)	64,000,000	43,000,000
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in investment and other income, net of tax	0	(13,000,000)	17,000,000
Other comprehensive (loss) income	(11,000,000)	51,000,000	60,000,000
Comprehensive income	$ 846,000,000	$ 1,382,000,000	$ 1,546,000,000