Consolidated Statement of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 477	$ 465
Other comprehensive income, net of tax:
Net unrealized gains on available-for-sale marketable securities, net of tax provision	11	9
Reclassification adjustment for net (gain) loss on available-for-sale marketable securities included in net income, net of tax	(2)	1
Other comprehensive income	9	10
Total comprehensive income	$ 486	$ 475