Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 291	$ 244	$ 655	$ 335
Other comprehensive (loss) income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities, net of tax	(6)	16	36	(38)
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in investment and other income, net of tax	(21)	9	(28)	25
Other comprehensive (loss) income	(27)	25	8	(13)
Comprehensive income	$ 264	$ 269	$ 663	$ 322