Segment information - Additional Information (Detail) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($) client	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) segment	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of reportable segments | segment				2
Concentration risk, number of customers				2
Revenues	[1]	$ 25,222.6	$ 25,778.5	$ 75,576.4	$ 74,574.5
Customer concentration risk | Revenue
Revenues from External Customers and Long-Lived Assets [Line Items]
Concentration risk, number of customers | client		2
Concentration risk, percentage of total revenue		28.70%		29.60%
Other Business Operations
Revenues from External Customers and Long-Lived Assets [Line Items]
Number of operating segments | segment				2
Revenues		$ 711.2	645.4	$ 2,054.4	1,856.8
Geographic Distribution, Foreign [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenues		19.8	$ 21.6	61.1	$ 65.5
Long-lived assets of international operations (consisting primarily of fixed assets)		$ 52.2		$ 52.2		$ 56.0

[1] Includes retail pharmacy co-payments of $2,161.5 million and $2,418.3 million for the three months ended September 30, 2015 and 2014, respectively, and $7,118.2 million and $7,894.7 million for the nine months ended September 30, 2015 and 2014, respectively