Significant Accounting Policies (Details)	3 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) segment	Mar. 31, 2018 USD ($) customer segment	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment		2
Restricted cash balance		$ 0	$ 1,500,000,000
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
LIFO inventory (percentage)		63.00%	70.00%
LIFO reserve		$ 906,000,000	$ 1,005,000,000
Charges (credits) associated with last-in-first-out inventory method		99,000,000	7,000,000	$ (244,000,000)
Capitalized software held for internal use, net		425,000,000	455,000,000
Capitalized software held for internal use, accumulated amortization		1,182,000,000	1,177,000,000
Sales returns from customers		3,100,000,000	3,100,000,000	3,100,000,000
Supplier reserves		227,000,000	201,000,000
Discrete tax benefits		$ 8,000,000	54,000,000	0
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		$ 914,000,000	814,000,000	789,000,000
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)		51.70%
Customer Concentration Risk | Sales Revenue, Net | CVS
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of total consolidated revenues (percent)		19.90%
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)		24.90%
Customer Concentration Risk | Accounts Receivable | CVS
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Percentage of total consolidated revenues (percent)		16.40%
New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Discrete tax benefits		$ 8,000,000	$ 54,000,000
Long-term Debt | Adjustments for New Accounting Pronouncement
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized debt issuance costs reclassified				$ 40,000,000
Expected
Accounting Policies [Abstract]
Number of operating segments | segment	3
Expected | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated deferred tax asset	$ 130,000,000
Expected | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Anticipated deferred tax asset	$ 160,000,000