WORKERS' PARTICIPATION: (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Statutory workers' participation
Current	$ 219.9	$ 192.7	$ 131.8
Deferred	9.8	6.6	(13.4)
Workers' participation expense (in USD)	$ 229.7	$ 199.3	$ 118.4
Peru
Statutory workers' participation
Provision for workers' participation as a percentage of pre-tax earnings	8.00%
Number of months for which an individual worker's salary is capped for consideration of annual individual worker participation	18 months
Maximum taxable units contributed by employer to benefit fund (in Peruvian taxable units) | item	2,200
Maximum employer contribution to benefit fund (in USD)	$ 2.7
Mexico
Statutory workers' participation
Provision for workers' participation as a percentage of pre-tax earnings	10.00%