Investments in Marketable Securities - Schedule of Gains and Losses on Investments in Marketable Securities Presented in Our Consolidated Statement of Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Available-for-sale debt securities:
Realized gains (losses)	$ 1	$ (1)	$ 1	$ (1)
Equity investments:
Unrealized gains (losses) recognized	31	(15)	86	(40)
Realized gains (losses) on sales	$ 1	$ 0	$ 3	$ 0