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Fair value measurements (Tables)
6 Months Ended
Jun. 30, 2011
Fair value measurements [Abstract]  
Carrying values and the fair values of Senior Notes
                                 
    June 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
  $ 1,494.1     $ 1,494.1     $     $  
5.25% senior notes due 2012, net of unamortized discount
    999.7       1,070.0       999.6       1,056.0  
6.25% senior notes due 2014, net of unamortized discount
    997.3       1,122.5       996.9       1,116.0  
7.25% senior notes due 2019, net of unamortized discount
    497.2       602.3       497.1       586.3  
     
Total
  $ 3,988.3     $ 4,288.9     $ 2,493.6     $ 2,758.3