Consolidated Statement of Comprehensive Income (Parentheticals) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net unrealized gains (losses) on available-for-sale marketable securities, tax (provision) benefit	$ (10)	$ (26)	$ 21	$ (35)
Reclassification adjustment for net losses (gains) on available-for-sale marketable securities included in net income, net of tax (benefit) provision	$ (5)	$ 1	$ (15)	$ 2