Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 307	$ 193	$ 962	$ 528
Other comprehensive (loss) income, net of tax:
Net unrealized gains (losses) on available-for-sale marketable securities, net of tax	5	2	41	(36)
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in investment and other income, net of tax	(7)	(7)	(35)	18
Other comprehensive (loss) income	(2)	(5)	6	(18)
Comprehensive income	$ 305	$ 188	$ 968	$ 510