Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 283	$ 432	$ 1,086	$ 1,348
Other comprehensive income, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities, net of tax	(22)	13	31	(8)
Reclassification adjustment for net losses (gains) on available-for-sale marketable securities included in net income, net of tax	0	2	(5)	17
Other comprehensive (loss) income	(22)	15	26	9
Total comprehensive income	$ 261	$ 447	$ 1,112	$ 1,357