Goodwill And Intangible Assets, Net - Information Regarding Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2015	Mar. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 5,220	$ 5,204
Accumulated Amortization	(1,966)	(1,763)
Net Carrying Amount	$ 3,254	3,441
Customer lists
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	9 years
Gross Carrying Amount	$ 2,686	2,683
Accumulated Amortization	(1,234)	(1,116)
Net Carrying Amount	$ 1,452	1,567
Service agreements
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	15 years
Gross Carrying Amount	$ 945	957
Accumulated Amortization	(240)	(215)
Net Carrying Amount	$ 705	742
Pharmacy licenses
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	26 years
Gross Carrying Amount	$ 897	874
Accumulated Amortization	(97)	(65)
Net Carrying Amount	$ 800	809
Trademarks and trade names
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	15 years
Gross Carrying Amount	$ 315	315
Accumulated Amortization	(90)	(82)
Net Carrying Amount	$ 225	233
Technology
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	3 years
Gross Carrying Amount	$ 209	213
Accumulated Amortization	(189)	(184)
Net Carrying Amount	$ 20	29
Other
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	4 years
Gross Carrying Amount	$ 168	162
Accumulated Amortization	(116)	(101)
Net Carrying Amount	$ 52	$ 61