Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of liabilities	$ 7,719		$ 7,880
Goodwill and asset impairment charges	570	$ 0
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	7,800		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	$ 165		$ 799