Summary of significant accounting policies (Details Textual 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Finite-Lived Intangible Assets [Line Items]
Amortization expense of other intangible assets	$ 236.0	$ 159.8	$ 114.6
Accumulated amortization of intangible assets	(593.3)	(383.6)
Amortization expense included in interest expense	81.0	5.1	66.3
PBM agreement with Well Point [Member]
Finite-Lived Intangible Assets [Line Items]
Estimated useful life of intangible assets (in years)	15
Amortization expense of other intangible assets	114.0	114.0	9.5
Accumulated amortization of intangible assets	(554.8)	(351.4)
Period of contract with WellPoint, years	10 years
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Major intangible class, minimum	5
Major intangible class, maximum	20
Customer Relationships [Member] | PBM agreement with Well Point [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense of other intangible assets	114.0	114.0	9.5
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Major intangible class, minimum	9
Major intangible class, maximum	30
Other Intangible Assets [Member] | PBM agreement with Well Point [Member]
Finite-Lived Intangible Assets [Line Items]
Accumulated amortization of intangible assets	(60.1)	(5.0)
Customer Related Intangibles and Non-compete Agreements [Member] | Selling, General and Administrative Expense [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization expense of other intangible assets	$ 40.7	$ 40.7	$ 34.7