Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 163	$ 265	$ 404	$ 400
Other comprehensive (loss) income, net of tax:
Net unrealized gains (losses) on available-for-sale marketable securities [net of tax (provision) benefit of ($10), $20, ($32) and $41, respectively]	16	(32)	52	(68)
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in investment and other income, net [net of tax provision (benefit) of $27, ($8), $32 and ($4), respectively]	(43)	13	(51)	6
Other comprehensive (loss) income	(27)	(19)	1	(62)
Comprehensive income	$ 136	$ 246	$ 405	$ 338