Changes in Business (Details Textual) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended				12 Months Ended				12 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2009	Nov. 14, 2011	Jun. 09, 2009	Dec. 31, 2011 External Customers [Member] Y	Dec. 31, 2011 Customer Contracts [Member]	Jun. 09, 2009 Unsecured credit facility for financing NextRx acquisition [Member]	Dec. 31, 2011 Express Scripts, Inc. [Member]	Dec. 31, 2011 Medco [Member]	Jun. 30, 2010 PBM agreement with Well Point [Member]	Dec. 31, 2011 PBM agreement with Well Point [Member] Y	Dec. 31, 2009 PBM agreement with Well Point [Member]
Business Acquisition [Line Items]
Percentage of voting rights company owned								59.00%	41.00%			100.00%
Total consideration description, equity interest issued or Issuable									$65.00 per share in cash and stock (valued based on the closing price of our stock on December 31, 2011), including $28.80 in cash and 0.81 shares for each Medco share owned
Total consideration for Medco shareholders									$ 25,900.0	$ 4,666.7
Cash and stock paid per share for Medco shareholders									$ 65.00
Cash paid per share for Medco shareholders									$ 28.80
Stock paid per share for Medco shareholders									$ 0.81
Purchase price adjustment for working capital										8.3
Total consideration for pharmacy benefit management services, paid in cash												4,675.0
Excess of purchase price over tangible net assets allocated to intangible assets					65.0	1,585.0					1,520.0
Estimated useful life of intangible assets (in years)					10						15
Amortization period to record greater portion of expense											First 5 years
Excess of purchase price over tangible net assets and identified intangible net assets allocated to goodwill											2,668.9
Transaction costs related to acquisition												61.1
Benefits Related To Amendment Of Client Contract										30.0
Senior notes aggregate principal amount			4,100	2,500			2,500
Share issued in common stock offering		52.9
Net proceeds resulting from public offering of common stock		1,569.1
Merger agreement maximum termination fee	950
Changes in Business (Textual) [Abstract]
Proceeds from issuance of senior notes	$ 4,100.0	$ 2,478.3