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Investments in Marketable Securities - Summary of Contractual Maturity of Available-for-Sale Debt Securities (Details) - USD ($)
$ in Millions
Sep. 30, 2019
Mar. 31, 2019
Available-for-sale debt securities, Amortized Cost    
Due within 1 Year, Amortized Cost $ 542  
Due after 1 Year through 5 Years, Amortized Cost 128  
Due after 5 Years through 10 Years, Amortized Cost 124  
Due after 10 Years, Amortized Cost 75  
Mortgage-backed and asset-backed securities, Amortized Cost [1] 156  
Available-for-sale debt securities, Amortized cost 1,025 $ 629
Available-for-sale debt securities, Fair Value    
Due within 1 Year, Fair Value 542  
Due after 1 Year through 5 Years, Fair Value 130  
Due after 5 Years through 10 Years, Fair Value 131  
Due after 10 Years, Fair Value 86  
Mortgage-backed and asset-backed securities, Fair Value [1] 160  
Total, Fair Value $ 1,049 $ 633
[1] Mortgage-backed and asset-backed securities are shown separately from other maturity groupings as these securities have multiple maturity dates.