Acquisitions (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended			9 Months Ended
		Aug. 18, 2015	Dec. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Business Acquisition [Line Items]
Outstanding common shares and voting interests acquired (percent)		100.00%
Common stock, par value				$ 0.01		$ 0.01		$ 0.01
Goodwill				$ 37,135		$ 37,135		$ 28,142
Goodwill acquired during period						9,035
Net revenues				38,644	$ 35,021	112,144	$ 102,312
Net loss				(1,247)	(948)	(3,740)	(3,323)
Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Share price (usd per share)		$ 98.00
Total consideration transferred		$ 9,645
Fair value of liabilities incurred		3,100
Fair value of restricted stock units issued		80
Fair value of replacement equity awards issued to Omnicare employees for precombination services		9
Current assets (including cash of $298)		1,682
Cash acquired		298
Property and equipment		314
Goodwill		9,035
Intangible assets		3,962
Other noncurrent assets		64
Current liabilities		(704)
Long-term debt		(3,110)
Deferred income tax liabilities		(1,533)
Other noncurrent liabilities		(65)
Goodwill amount expected to be deductible for tax purposes		$ 400
Acquired intangible assets' weighted average useful lives		18 years 9 months
Business acquisition transaction costs				52		68
Net revenues				710		710
Net loss				3		3
Restricted Stock [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Restricted stock units issued (in shares)		738,765
Cash and Cash Equivalents [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Cash paid to Omnicare shareholders		$ 9,636
Retail/LTC Segment [Member]
Business Acquisition [Line Items]
Goodwill				15,458		15,458		6,908
Goodwill acquired during period						8,591
Net revenues				17,912	16,749	52,105	50,100
Retail/LTC Segment [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Goodwill acquired during period						8,591
Pharmacy Services Segment
Business Acquisition [Line Items]
Goodwill				21,677		21,677		$ 21,234
Goodwill acquired during period						444
Net revenues	[1]			25,528	$ 22,534	73,849	$ 64,566
Pharmacy Services Segment | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Goodwill acquired during period						444
Customer Relationships [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets		$ 3,900
Acquired intangible assets' weighted average useful lives		19 years 1 month
Trade Names [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Intangible assets		$ 74
Acquired intangible assets' weighted average useful lives		2 years 11 months
Other Acquired Receivables [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Fair value of trade accounts receivable				147		147
Gross contractual amount of accounts receivable				161		161
TRade accounts receivable expected to be uncollectible				14		14
Trade Accounts Receivable [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Fair value of trade accounts receivable				600		600
Gross contractual amount of accounts receivable				857		857
TRade accounts receivable expected to be uncollectible				$ 257		$ 257
Pro Forma [Member] | Omnicare, Inc. [Member]
Business Acquisition [Line Items]
Cash paid to Omnicare shareholders			$ 1,887

[1] Net revenues of the Pharmacy Services Segment include approximately $2.1 billion and $1.9 billion of retail co-payments for the three months ended September 30, 2015 and 2014, respectively, as well as $6.8 billion and $6.1 billion of retail co-payments for the nine months ended September 30, 2015 and 2014, respectively.