Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 342	$ 307	$ 742	$ 962
Other comprehensive (loss) income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities net of tax	(23)	5	(91)	41
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in investment and other income, net of tax	0	(7)	6	(35)
Other comprehensive (loss) income	(23)	(2)	(85)	6
Comprehensive income	$ 319	$ 305	$ 657	$ 968