Fair Value Measurements (Details)	3 Months Ended		9 Months Ended
	Dec. 31, 2020 USD ($) investmentSecurity	Sep. 30, 2020 company	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	$ 267,000,000		$ 267,000,000	$ 170,000,000
Number of companies in which equity securities were converted into public common stock | company		3
Number of investments that experienced transaction that resulted in change in observable price of those securities | investmentSecurity	2
Unrealized gain (loss) on equity investments	$ 28,000,000		87,000,000
Recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of liabilities	7,200,000,000		7,200,000,000	7,400,000,000
Estimated fair values of liabilities	8,000,000,000.0		8,000,000,000.0	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	123,000,000		123,000,000	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