Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of liabilities	$ 7,600,000,000	$ 7,900,000,000
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities measured at fair value on a nonrecurring basis	0
Fair Value, Measurements, Nonrecurring | Intangible Asset and Store Assets Impairment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Pre-tax charge	245,000,000	479,000,000
Restructuring charges, after tax	207,000,000	443,000,000
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of liabilities	7,900,000,000	8,100,000,000
Level 1 | Fair Value, Measurements, Recurring | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 1,205,000,000	$ 799,000,000