Segment information (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Reportable Segments Information
Network revenues	$ 7,389.0		$ 7,590.6		$ 14,647.1		$ 15,105.5
Home delivery and specialty revenues	3,518.6		3,295.2		6,925.7		6,525.8
Other revenues	382.6		334.2		733.1		658.2
Service revenues	71.2		68.8		150.0		137.7
Total revenues	11,361.4	[1]	11,288.8	[1]	22,455.9	[1]	22,427.2	[1]
Depreciation and amortization expense	63.3		60.5		126.2		119.2
Operating income	579.3		530.3		1,131.7		985.0
Interest income	1.5		0.5		1.9		2.2
Interest expense	(50.3)		(42.0)		(90.0)		(84.8)
Income before income taxes	530.5		488.8		1,043.6		902.4
Capital expenditures	34.6		17.4		54.1		51.1
PBM [Member]
Reportable Segments Information
Network revenues	7,389.0		7,590.6		14,647.1		15,105.5
Home delivery and specialty revenues	3,518.6		3,295.2		6,925.7		6,525.8
Service revenues	65.9		65.8		139.4		131.7
Total revenues	10,973.5		10,951.6		21,712.2		21,763.0
Depreciation and amortization expense	61.2		58.5		122.0		115.2
Operating income	569.2		525.4		1,113.3		976.0
Capital expenditures	33.7		16.3		52.8		49.8
EM [Member]
Reportable Segments Information
Other revenues	382.6		334.2		733.1		658.2
Service revenues	5.3		3.0		10.6		6.0
Total revenues	387.9		337.2		743.7		664.2
Depreciation and amortization expense	2.1		2.0		4.2		4.0
Operating income	10.1		4.9		18.4		9.0
Capital expenditures	$ 0.9		$ 1.1		$ 1.3		$ 1.3

[1] Includes retail pharmacy co-payments of $1,457.1 million and $1,547.3 million for the three months ended June 30, 2011 and 2010, respectively and $2,983.6 million and $3,209.9 million for the six months ended June 30, 2011 and 2010, respectively.