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Fair value measurements (Tables)
9 Months Ended
Sep. 30, 2011
Fair value measurements [Abstract] 
Carrying values and the fair values of Senior Notes
                                 
    September 30, 2011     December 31, 2010  
    Carrying     Fair     Carrying     Fair  
(in millions)   Amount     Value     Amount     Value  
 
3.125% senior notes due 2016, net of unamortized discount
  $ 1,494.4     $ 1,492.1     $     $  
5.25% senior notes due 2012, net of unamortized discount
    999.8       1,027.5       999.6       1,056.0  
6.25% senior notes due 2014, net of unamortized discount
    997.6       1,095.2       996.9       1,116.0  
7.25% senior notes due 2019, net of unamortized discount
    497.2       607.6       497.1       586.3  
     
Total
  $ 3,989.0     $ 4,222.4     $ 2,493.6     $ 2,758.3