Consolidated Statement of Comprehensive Income (Parentheticals) (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]
Unrealized gains (losses) on available-for-sale marketable securities, tax (provision) benefit	$ (26)	$ 19	$ (35)	$ 16
Reclassification adjustment for net gain (loss) on available-for-sale marketable securities included in net income, tax provision (benefit)	$ 1	$ (8)	$ 2	$ (9)