Segment information (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Reportable Segments Information
Network revenues	$ 7,491.0		$ 7,490.9		$ 22,138.1		$ 22,596.4
Home delivery and specialty revenues	3,678.4		3,369.0		10,715.6		9,990.0
Other revenues	330.6		326.3		952.2		889.3
Service revenues	71.0		65.6		221.0		203.3
Total revenues	11,571.0	[1]	11,251.8	[1]	34,026.9	[1]	33,679.0	[1]
Depreciation and amortization expense	61.3		61.7		187.5		180.9
Operating income	605.1		528.0		1,736.8		1,513.0
Interest income	5.9		2.2		7.8		4.4
Interest expense and other	(94.3)		(42.2)		(184.3)		(127.0)
Income before income taxes	516.7		488.0		1,560.3		1,390.4
Capital expenditures	44.0		33.5		98.1		84.6
PBM [Member]
Reportable Segments Information
Network revenues	7,491.0		7,490.9		22,138.1		22,596.4
Home delivery and specialty revenues	3,678.4		3,369.0		10,715.6		9,990.0
Service revenues	65.7		62.6		205.1		194.3
Total revenues	11,235.1		10,922.5		33,058.8		32,780.7
Depreciation and amortization expense	59.3		59.8		181.5		175.1
Operating income	600.8		525.5		1,719.3		1,505.6
Capital expenditures	42.8		32.7		95.6		82.7
EM [Member]
Reportable Segments Information
Other revenues	330.6		326.3		952.2		889.3
Service revenues	5.3		3.0		15.9		9.0
Total revenues	335.9		329.3		968.1		898.3
Depreciation and amortization expense	2.0		1.9		6.0		5.8
Operating income	4.3		2.5		17.5		7.4
Capital expenditures	$ 1.2		$ 0.8		$ 2.5		$ 1.9

[1] Includes retail pharmacy co-payments of $1,390.4 million and $1,478.5 million for the three months ended September 30, 2011 and 2010, respectively and $4,374.0 million and $4,688.4 million for the nine months ended September 30, 2011 and 2010, respectively.