Quarterly financial data (unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Unaudited quarterly financial data
Total revenues	$ 12,101.4	$ 11,571.0	$ 11,361.4	$ 11,094.5	$ 11,294.2	$ 11,251.8	$ 11,288.8	$ 11,138.4	$ 46,128.3	[1]	$ 44,973.2	[1]	$ 24,722.3	[1]
Cost of revenues	11,256.9	10,735.2	10,577.3	10,349.0	10,520.8	10,487.7	10,531.3	10,475.2	42,918.4	[1]	42,015.0	[1]	22,298.3	[1]
Gross profit	844.5	835.8	784.1	745.5	773.4	764.1	757.5	663.2	3,209.9		2,958.2		2,424.0
Selling, general and administrative	269.6	230.7	204.8	193.1	215.5	236.1	227.2	208.5	898.2		887.3		926.5
Operating income	574.9	605.1	579.3	552.4	557.9	528.0	530.3	454.7	2,311.7		2,070.9		1,497.5
Net income from continuing operations					329.6	307.1	307.3	260.6	1,275.8		1,204.6		826.6
Net (loss) income from discontinued operations, net of tax						(5.6)	(17.4)	(0.4)	0		(23.4)		1.0
Net income	290.4	324.7	334.2	326.5	329.6	301.5	289.9	260.2	1,275.8		1,181.2		827.6
Basic earnings (loss) per share:
Continuing operations					$ 0.62	$ 0.58	$ 0.56	$ 0.47	$ 2.55		$ 2.24		$ 1.57
Discontinued operations						$ (0.01)	$ (0.03)				$ (0.04)
Basic earnings per share	$ 0.60	$ 0.67	$ 0.66	$ 0.62	$ 0.62	$ 0.57	$ 0.53	$ 0.47	$ 2.55		$ 2.19		$ 1.57
Diluted earnings (loss) per share:
Continuing operations					$ 0.62	$ 0.57	$ 0.56	$ 0.47	$ 2.53		$ 2.21		$ 1.55
Discontinued operations						$ (0.01)	$ (0.03)				$ (0.04)
Diluted earnings per share	$ 0.59	$ 0.66	$ 0.66	$ 0.61	$ 0.62	$ 0.56	$ 0.53	$ 0.47	$ 2.53		$ 2.17		$ 1.56
Quarterly financial data (unaudited) (Textual) [Abstract]
Retail pharmacy co-payments included in network revenues	$ 1,412.6	$ 1,390.4	$ 1,457.1	$ 1,526.5	$ 1,493.0	$ 1,478.5	$ 1,547.3	$ 1,662.6	$ 5,786.6		$ 6,181.4		$ 3,132.1

[1] Includes retail pharmacy co-payments of $5786.6, $6,181.4, and $3,132.1, for the years ended December 31, 2011, 2010, and 2009, respectively.