ASSET RETIREMENT OBLIGATION: (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended	1 Months Ended	51 Months Ended
	Mar. 31, 2014 item Y	Mar. 31, 2013	Dec. 31, 2012	Jan. 31, 2014 Mexican operations	Mar. 31, 2014 Mexican operations
ASSET RETIREMENT OBLIGATION:
Period after which successive reviews are required by the law (in years)	5
Accepted value of the Lima office building	$ 27.8
Cumulative guarantee amount	14.2
Number of units with future closure costs recognized as an asset retirement obligation	3
Environmental costs
Increase in plant demolition and soil remediation budgeted cost				26.7
Plant demolition and soil remediation budgeted cost					62.4
Plant demolition and soil cost recognized					36.4
Increase to net property due to closure of mine			20.3
Asset retirement obligation activity
Balance at the beginning of the period	124.8	122.3
Changes in estimates	26.7
Additions			25.1
Payments	(1.5)	(1.9)
Accretion expense	2.0	1.8
Balance at the end of the period	$ 152.0	$ 122.2	$ 122.3