COMMITMENTS AND CONTINGENCIES: - Environmental matters (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2011 category person
Environmental costs
Environmental capital investment	$ 12.4	$ 19.2
Peru
Environmental costs
Environmental capital investment	2.5	10.2
Mexico
Environmental costs
Environmental capital investment	$ 9.9	$ 9.0
Number of categories of collective actions | category			3
Minimum number of people claiming injury due to collective action initiative in Civil Federal Procedures Code (CFPC) | person			30