Goodwill and Intangible Assets, Net - Schedule of Information Regarding Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2018	Mar. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 7,194	$ 6,681
Accumulated Amortization	(3,066)	(2,579)
Net Carrying Amount	$ 4,128	4,102
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	12 years
Gross Carrying Amount	$ 4,036	3,619
Accumulated Amortization	(1,797)	(1,550)
Net Carrying Amount	$ 2,239	2,069
Service agreements
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	12 years
Gross Carrying Amount	$ 1,027	1,037
Accumulated Amortization	(406)	(386)
Net Carrying Amount	$ 621	651
Pharmacy licenses
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	25 years
Gross Carrying Amount	$ 777	684
Accumulated Amortization	(357)	(196)
Net Carrying Amount	$ 420	488
Trademarks and trade names
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	14 years
Gross Carrying Amount	$ 925	932
Accumulated Amortization	(223)	(187)
Net Carrying Amount	$ 702	745
Technology
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	4 years
Gross Carrying Amount	$ 141	147
Accumulated Amortization	(86)	(84)
Net Carrying Amount	$ 55	63
Other
Finite-Lived Intangible Assets [Line Items]
Weighted Average Remaining Amortization Period (years)	5 years
Gross Carrying Amount	$ 288	262
Accumulated Amortization	(197)	(176)
Net Carrying Amount	$ 91	$ 86