Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Mar. 31, 2017 USD ($) customer segment	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment	2
Restricted cash balance	$ 1,500	$ 939
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
LIFO inventory (percentage)	70.00%	74.00%
LIFO reserve	$ 1,005	$ 1,012
Charges (credits) associated with last-in-first-out inventory method	$ 7	(244)	$ (337)
Capitalized software held for sale, useful life	3 years
Capitalized software held for internal use, net	$ 455	435
Capitalized software held for internal use, accumulated amortization	1,177	1,130
Sales returns from customers	3,100	3,100	2,700
Supplier reserves	201	144
Discrete tax benefits	$ 54	0	0
New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Current net deferred tax liabilities reclassified		2,000
Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	1 year
Intangible assets, useful life	1 year
Capitalized software held for internal use, useful life	1 year
Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property, plant and equipment, useful life	30 years
Intangible assets, useful life	38 years
Capitalized software held for internal use, useful life	10 years
Selling, Distribution and Administrative Expenses
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shipping and handling costs	$ 814	789	$ 819
Customer Concentration Risk | Sales Revenue, Net
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of largest customers | customer	10
Percentage of total consolidated revenues (percent)	54.20%
Customer Concentration Risk | Sales Revenue, Net | CVS
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of total consolidated revenues (percent)	20.20%
Customer Concentration Risk | Accounts Receivable
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of largest customers | customer	10
Percentage of total consolidated revenues (percent)	33.70%
Customer Concentration Risk | Accounts Receivable | CVS
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of total consolidated revenues (percent)	17.80%
New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Discrete tax benefits	$ 54
Long-term Debt [Member] | Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt issuance costs reclassified		$ 40