Consolidated Statement of Comprehensive Income (Parentheticals) (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]
Unrealized gains (losses) on available-for-sale marketable securities, tax (provision) benefit	$ 13	$ (9)	$ (22)	$ 7
Reclassification adjustment for net losses (gains) on available-for-sale marketable securities included in net income, tax provision (benefit)	$ 0	$ (2)	$ 2	$ (10)