Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 194	$ 117	$ 286	$ 282
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on available-for-sale marketable securities [net of tax provision of ($1), ($5), ($1) and ($15), respectively]	1	9	(3)	26
Reclassification adjustment for net losses (gains) on available-for-sale marketable securities included in investment and other income, net [net of tax provision of $0, $0, $0 and $16, respectively]	1	(1)	1	(26)
Other comprehensive income (loss)	2	8	(2)	0
Comprehensive income	$ 196	$ 125	$ 284	$ 282