Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2016	May 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Book Value	$ 502		$ 1,200	$ 1,300
Repayments of Accounts Receivable Securitization	326
Short-term Investments	$ 1,650		0
Document Period End Date	Sep. 30, 2016
Marketable Securities, Equity Securities	$ 51		$ 46
MLS 2nd Tranche [Member]
Sale Leaseback Transaction [Line Items]
Sale Leaseback Transaction, Net Book Value	758	$ 1,300
Sale Leaseback Transaction, Current Period Gain Recognized	$ 30