Segment information (Details Textual) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment information (Textual) [Abstract]
Percentage threshold of total revenue represented by top clients	10% or more	10% or more	10% or more
Percentage of threshold revenue	10.00%	10.00%	10.00%
Number of clients accounted for revenue above threshold	None	None	None
Revenues earned by Canadian PBM	$ 62.4	$ 52.2	$ 49.2
Long-lived assets of Canadian PBM (consisting primarily of fixed assets)	$ 17.6	$ 16.7

Amount of long-lived assets other than financial instruments, long-term customer relationships of a financial institution, mortgage and other servicing rights, deferred policy acquisition costs, and deferred tax assets, located in foreign countries in total in which the entity holds assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 38

-Subparagraph b(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 280

-SubTopic 10

-Section 50

-Paragraph 41

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e9038-108599

Name:	us-gaap_EntityWideDisclosureOnGeographicAreasLongLivedAssetsInForeignCountries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant