Cash and Cash Equivalents and 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	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Available-for-sale securities:
Realized (losses) gains	$ (12)	$ 41	$ 226
Equity investments:
Unrealized gains recognized	$ 14