Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 326	$ 451	$ 803	$ 916
Other comprehensive income, net of tax:
Net unrealized gains (losses) on available-for-sale marketable securities, net of tax	42	(30)	53	(21)
Reclassification adjustment for net (gains) losses on available-for-sale marketable securities included in net income, net of tax	(3)	14	(5)	15
Other comprehensive income (loss)	39	(16)	48	(6)
Total comprehensive income	$ 365	$ 435	$ 851	$ 910