Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 241	$ 135
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on available-for-sale marketable securities, net of tax	36	(36)
Reclassification adjustment for net gains on available-for-sale marketable securities included in investment and other income, net, net of taxes	(8)	(7)
Other comprehensive income (loss)	28	(43)
Comprehensive income	$ 269	$ 92