Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 265	$ 291	$ 400	$ 655
Other comprehensive (loss) income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities net of tax	(32)	(6)	(68)	36
Reclassification adjustment for net losses (gains) on available-for-sale marketable securities included in investment and other income, net of tax	13	(21)	6	(28)
Other comprehensive (loss) income	(19)	(27)	(62)	8
Comprehensive income	$ 246	$ 264	$ 338	$ 663