Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 165	$ 241
Other comprehensive (loss) income, net of tax:
Net unrealized gains on available-for-sale marketable securities [net of tax provision of ($10) and ($22), respectively]	17	36
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 $16 and $5, respectively]	(25)	(8)
Other comprehensive (loss) income	(8)	28
Comprehensive income	$ 157	$ 269