Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 3,076	$ (46)	$ 3,358	$ 358
Other comprehensive loss, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities [net of tax benefit (provision) of $8, $40, ($7) and $8, respectively]	(13)	(66)	13	(14)
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 $1, $60, $17 and $92, respectively]	(2)	(97)	(28)	(148)
Other comprehensive loss	(15)	(163)	(15)	(162)
Comprehensive income (loss)	$ 3,061	$ (209)	$ 3,343	$ 196