Business Combinations (Details) € in Millions, £ in Millions, $ in Millions, CAD in Billions						1 Months Ended	3 Months Ended	6 Months Ended
	Aug. 31, 2016 GBP (£)	Aug. 31, 2016 USD ($)	Apr. 01, 2016 EUR (€) cancer_treatment_facility state	Apr. 01, 2016 USD ($) cancer_treatment_facility state	Feb. 29, 2016 USD ($)	Mar. 31, 2016 CAD	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2016 USD ($) pharmacy
Business Acquisition [Line Items]
Net purchase consideration								$ 2,041	$ 11
Goodwill							$ 9,693	9,693		$ 9,786
Vantage
Business Acquisition [Line Items]
Number of cancer treatment facilities acquired | cancer_treatment_facility			51	51
Number of states | state			13	13
Net purchase consideration				$ 515
Goodwill							512	512
Fair value of noncontrolling interests							89	89
Intangible assets							15	15
Vantage | Noncompete Agreements
Business Acquisition [Line Items]
Intangible assets							7	$ 7
Estimated weighted average life of the acquired intangibles								4 years
Biologics
Business Acquisition [Line Items]
Net purchase consideration				692
Goodwill							566	$ 566
Intangible assets							193	193
Biologics | Trademarks
Business Acquisition [Line Items]
Intangible assets							170	$ 170
Estimated weighted average life of the acquired intangibles								9 years
Vantage and Biologics
Business Acquisition [Line Items]
Goodwill							1,078	$ 1,078
Fair value of noncontrolling interests							89	89
Intangible assets							208	208
UDG
Business Acquisition [Line Items]
Net purchase consideration			€ 380	$ 431
Increase in goodwill recognized							20
Total assets acquired, excluding goodwill and intangibles							473	473
Total liabilities acquired, excluding goodwill and intangibles							342	342
UDG | Customer relationships
Business Acquisition [Line Items]
Intangible assets							$ 115	$ 115
Estimated weighted average life of the acquired intangibles								10 years
Sainsbury | U.K.
Business Acquisition [Line Items]
Net purchase consideration	£ 125	$ 164
Total assets acquired, excluding goodwill and intangibles		30
Total liabilities acquired, excluding goodwill and intangibles		10
Gross purchase consideration paid					$ 174
Interest rate (percent)					0.033
Sainsbury | Pharmacy licenses | U.K.
Business Acquisition [Line Items]
Intangible assets		$ 65
Estimated weighted average life of the acquired intangibles	15 years	15 years
Rexall Health | CANADA
Business Acquisition [Line Items]
Number of cancer treatment facilities acquired | pharmacy										470
Gross purchase consideration paid						CAD 3		$ 2,300
Currency exchange ratio (in CAD per USD)							0.76	0.76