Fair Value Measurements (Details)	3 Months Ended	6 Months Ended
	Sep. 30, 2020 USD ($) company	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	$ 235,000,000	$ 235,000,000	$ 170,000,000
Number of companies in which equity securities were converted into public common stock | company	3
Unrealized gain (loss) on equity investments	$ 49,000,000	59,000,000
Recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of liabilities	7,600,000,000	7,600,000,000	7,400,000,000
Estimated fair values of liabilities	8,400,000,000	8,400,000,000	7,800,000,000
Recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in money market funds	129,000,000	129,000,000	2,000,000,000.0
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	$ 0	$ 0	$ 0