Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 92	$ 165
Other comprehensive loss, net of tax:
Net unrealized (losses) gains on available-for-sale marketable securities [net of tax provision of $0 and ($10), respectively]	(4)	17
Reclassification adjustment for net gains on available-for-sale marketable securities included in investment and other income, net [net of tax provision of $0 and $16, respectively]	0	(25)
Other comprehensive loss	(4)	(8)
Comprehensive income	$ 88	$ 157