Debt and Financing Activities - Long Term Debt (Details) £ in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2015 GBP (£)	Mar. 31, 2015 USD ($)
Debt Instrument [Line Items]
Long-term debt outstanding	$ 9,200		$ 9,200			$ 9,700
Repayment of term loan			$ 498	$ 231
Repayment of floating rate notes	$ 400
Celesio | Bonds
Debt Instrument [Line Items]
Long-term debt outstanding					£ 60	89
Repayment of term loan		$ 93
Celesio | 4.00% bonds due October 18, 2016
Debt Instrument [Line Items]
Euro denominated corporate bonds (as a percent)	4.00%		4.00%
Long-term debt outstanding	$ 400		$ 400			388
Celesio | 4.00% bonds due October 18, 2016 | Bonds
Debt Instrument [Line Items]
Long-term debt outstanding	$ 980		$ 980
Celesio | 4.50% bonds due April 26, 2017
Debt Instrument [Line Items]
Euro denominated corporate bonds (as a percent)	4.50%		4.50%
Long-term debt outstanding	$ 580		$ 580			563
Celesio | 4.50% bonds due April 26, 2017 | Bonds
Debt Instrument [Line Items]
Long-term debt outstanding						$ 951