Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of liabilities	$ 7,694			$ 7,694		$ 7,880
Goodwill and asset impairment charges	0	$ 570	$ 350	570	$ 350
Fair malue, measurements, recurring | Fair value, inputs, level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair values of liabilities	7,700			7,700		8,100
Fair malue, measurements, recurring | Fair value, inputs, level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in money market funds	$ 527			$ 527		$ 799