Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 117	$ 163	$ 282	$ 404
Other comprehensive income (loss), net of tax:
Net unrealized gains on available-for-sale marketable securities [net of tax provision of ($5), ($10), ($15) and ($32), respectively]	9	16	26	52
Reclassification adjustment for net gains on available-for-sale marketable securities included in realized gains, net on investments in marketable securities [net of tax provision of $0, $27, $16 and $32, respectively]	(1)	(43)	(26)	(51)
Other comprehensive income (loss)	8	(27)	0	1
Comprehensive income	$ 125	$ 136	$ 282	$ 405