INCOME TAXES: (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Taxes
Statutory income tax provision			$ 694.7	$ 648.0
Peruvian royalty			14.1	8.6
Mexican royalty			44.1	47.6
Peruvian special mining tax			31.8	25.8
Total provision for income taxes	$ 338.5	$ 241.0	$ 784.7	$ 730.0
Effective income tax rate (as a percent)			44.40%	38.20%
Changes in the entity's uncertain tax positions			$ 7.5
Peru
Income Taxes
Peruvian royalty			14.1	$ 8.6
Royalty charges			39.7	31.7
Accrued special mining tax			31.8	25.8
Mexico
Income Taxes
Mexican royalty			$ 44.1	$ 47.6