Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (46)	$ 342	$ 358	$ 742
Other comprehensive (loss) income, net of tax:
Net unrealized losses on available-for-sale marketable securities [net of tax benefit of $40, $13, $8 and $54, respectively]	(66)	(23)	(14)	(91)
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 $60, $0, $92 and ($4), respectively]	(97)	0	(148)	6
Other comprehensive (loss) income	(163)	(23)	(162)	(85)
Comprehensive (loss) income	$ (209)	$ 319	$ 196	$ 657