ASSET RETIREMENT OBLIGATION: (Details) $ in Thousands	3 Months Ended		9 Months Ended		69 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)
ASSET RETIREMENT OBLIGATION:
Period after which successive reviews are required by the law (in years)			5 years
Accepted value of the Lima office building	$ 27,800		$ 27,800		$ 27,800
Cumulative guarantee amount	17,900		$ 17,900		17,900
Number of units with future closure costs recognized as an asset retirement obligation | item			3
Environmental costs
Environmental remediation	7,043	$ 37,163	$ 23,503	$ 37,163
Asset retirement obligation activity
Balance at the beginning of the period			116,100	124,800
Changes in estimates				26,700
Closure payments			(15,300)	(8,100)
Accretion expense			13,100	5,800
Balance at the end of the period	$ 113,900	$ 149,200	$ 113,900	$ 149,200	113,900
Mexico
Environmental costs
Plant demolition and soil remediation budgeted cost					62,400
Environmental remediation					$ 62,200