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ASSET RETIREMENT OBLIGATION: (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2013
item
Jun. 30, 2012
ASSET RETIREMENT OBLIGATION:    
Period after which the law requires a first review 3 years  
Period after which successive reviews are required by the law 5 years  
Accepted value of the Lima office building $ 17  
Cumulative guarantee amount 10.5  
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 118.2 62.0
Closure payments (0.7)  
Accretion expense 4.9 1.7
Balance at the end of the period $ 122.4 $ 63.7