Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 3,410	$ 267	$ 932
Other comprehensive loss, net of tax:
Net unrealized losses on available-for-sale marketable securities [net of tax benefit of $6, $0 and $32, respectively]	(29)	(1)	(51)
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 $16, $87 and $2, respectively]	(25)	(139)	(4)
Other comprehensive loss	(54)	(140)	(55)
Comprehensive income	$ 3,356	$ 127	$ 877