Investments in Marketable Securities - Summary of Contractual Maturity of Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions		Sep. 30, 2020	Mar. 31, 2020
Available-for-sale debt securities, Amortized Cost
Due within 1 Year, Amortized Cost		$ 399
Due after 1 Year through 5 Years, Amortized Cost		118
Due after 5 Years through 10 Years, Amortized Cost		142
Due after 10 Years, Amortized Cost		73
Mortgage-backed and asset-backed securities, Amortized Cost	[1]	144
Available-for-sale debt securities, Amortized cost		$ 876	$ 1,381
Debt Securities, Available-for-sale, Maturity Allocation [Extensible List]		us-gaap:DebtSecuritiesMaturityAllocationContractualMaturityMember
Available-for-sale debt securities, Fair Value
Due within 1 Year, Fair Value		$ 399
Due after 1 Year through 5 Years, Fair Value		125
Due after 5 Years through 10 Years, Fair Value		154
Due after 10 Years, Fair Value		83
Mortgage-backed and asset-backed securities, Fair Value	[1]	148
Total, Fair Value		$ 909	$ 1,401
Debt Securities, Available-for-sale, Maturity Allocation [Extensible List]		us-gaap:DebtSecuritiesMaturityAllocationContractualMaturityMember

[1] Mortgage-backed and asset-backed securities are shown separately from other maturity groupings as these securities have multiple maturity dates.