Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	$ 357,000,000		$ 357,000,000		$ 269,000,000
Unrealized gain (loss) on equity investments	97,000,000	$ 49,000,000	104,000,000	$ 59,000,000
National Prescription Opioid Litigation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted cash	354,000,000		354,000,000
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value	0		0		0
Fair value, inputs, level 1 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in money market funds	$ 361,000,000		$ 361,000,000		$ 1,600,000,000