Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 267	$ 932	$ 1,197
Other comprehensive (loss) income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities [net of tax benefit (provision) of $0, $32 and ($38), respectively]	(1)	(51)	64
Reclassification adjustment for net gains on available-for-sale marketable securities included in realized gains, net on investments in marketable securities [net of tax provision of $87, $2 and $26, respectively]	(139)	(4)	(44)
Other comprehensive (loss) income	(140)	(55)	20
Comprehensive income	$ 127	$ 877	$ 1,217