Business Combinations (Details) $ in Millions						1 Months Ended	3 Months Ended			6 Months Ended		9 Months Ended
	Jun. 01, 2018 USD ($)	May 23, 2018 USD ($)	May 23, 2018 CAD ($)	Jan. 02, 2018 USD ($)	Apr. 03, 2017 USD ($)	May 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) pharmacy	Dec. 31, 2016 CAD ($) pharmacy	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) pharmacy	Dec. 31, 2017 CAD ($) pharmacy	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 CAD ($)
Business Acquisition [Line Items]
Goodwill										$ 10,627,000,000					$ 10,924,000,000
Net purchase consideration										840,000,000	$ 1,874,000,000
Medical Specialties Distributors LLC (“MSD”)
Business Acquisition [Line Items]
Consideration transferred to acquire business	$ 784,000,000
Assets acquired as of the acquisition date, excluding goodwill and intangibles	244,000,000
Liabilities assumed as of the acquisition date	161,000,000
Goodwill	375,000,000
Intangible assets	$ 326,000,000
Weighted average life of intangibles	18 years
Rexall Health | CANADA
Business Acquisition [Line Items]
Net purchase consideration								$ 2,100,000,000	$ 2,900
Number of retail pharmacies | pharmacy								413	413
Number of Stores Sold | pharmacy												27	27
RxCrossroads
Business Acquisition [Line Items]
Assets acquired as of the acquisition date, excluding goodwill and intangibles				$ 128,000,000
Liabilities assumed as of the acquisition date				56,000,000
Goodwill				386,000,000
Intangible assets				$ 262,000,000
Weighted average life of intangibles				14 years
Net purchase consideration				$ 720,000,000
CoverMyMeds, LLC (CMM)
Business Acquisition [Line Items]
Net purchase consideration					$ 1,300,000,000
intraFUSION, BDI Pharma, LLC, and Uniprix Group
Business Acquisition [Line Items]
Assets acquired as of the acquisition date, excluding goodwill and intangibles										292,000,000
Liabilities assumed as of the acquisition date										160,000,000
Goodwill										246,000,000
Intangible assets										118,000,000
Net purchase consideration							$ 485,000,000
Additional amount, Up to | CoverMyMeds, LLC (CMM)
Business Acquisition [Line Items]
Contingent consideration														$ 160,000,000
Estimate of Fair Value | CoverMyMeds, LLC (CMM)
Business Acquisition [Line Items]
Contingent consideration					$ 113,000,000					$ 64,000,000					$ 124,000,000
Payment for contingent consideration liability						$ 68,000,000
Third Party Buyer of Rexall Health | Rexall Health | CANADA
Business Acquisition [Line Items]
Net cash proceeds received												$ 94,000,000				$ 116
Third Party Seller of Rexall Health | Rexall Health | CANADA
Business Acquisition [Line Items]
Proceeds from prior acquisitions		$ 97,000,000	$ 125									119,000,000	$ 147
Gain (loss) recognized on sale of stores												$ 0