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ASSET RETIREMENT OBLIGATION: (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Y
unit
Dec. 31, 2010
ASSET RETIREMENT OBLIGATION:    
Annual Guarantees $ 2.6  
Period of annual installments (in years) 34  
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
Changes in estimates   5.3
Additions   1.6
Closure payments (0.5)  
Asset retirement obligation, accretion expense 3.4 3.3
Balance at the end of the period $ 62.0 $ 59.1