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ASSET RETIREMENT OBLIGATION: (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
ASSET RETIREMENT OBLIGATION:    
Annual installments $ 2.6  
Period of annual installments (in years) 34  
Cumulative installments 5.2  
Accepted value of the Lima office building 17  
Number of units with future closure costs recognized as an asset retirement obligation 3  
Asset retirement obligation activity    
Balance at the beginning of the period 59.1 48.9
Change in estimates   8.7
Accretion expense 1.7 1.6
Balance at the end of the period $ 60.8 $ 59.2