INCOME TAXES: (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Taxes
Statutory income tax provision			$ 289.1	$ 175.5
Peruvian royalty			0.4
Mexican royalty			32.3	13.7
Peruvian special mining tax			14.8	5.0
Total income tax provision	$ 160.3	$ 102.0	$ 336.6	$ 194.2
Effective income tax rate (as a percent)			35.70%	32.70%
Changes in the entity's uncertain tax positions			$ 0.0
Peru
Income Taxes
Peruvian royalty			$ 0.4	$ 0.0
Increment in operating income margin for royalty tax (as a percent)			5.00%
Increase in royalty tax rate for each 5% increase up to 12% increase in operating income margin (as a percent)			0.75%
Royalty charges			$ 10.4	8.1
Increase in special mining tax rate for each 5% increase up to 85% increase in operating income margin (as a percent)			0.40%
Increment in operating income margin (as a percent)			5.00%
Accrued special mining tax			$ 14.8	5.0
Peru | Minimum
Income Taxes
Mining royalty tax (as a percent)			1.00%
Royalty charge assessed as a percentage of net sales			1.00%
Special mine tax (as a percent)			2.00%
Increment in operating income margin (as a percent)			2.00%
Peru | Maximum
Income Taxes
Mining royalty tax (as a percent)			12.00%
Operating income margin for royalty tax (as a percent)			10.00%
Increment in operating income margin for royalty tax (as a percent)			12.00%
Special mine tax (as a percent)			8.40%
Operating income margin (as a percent)			10.00%
Increment in operating income margin (as a percent)			85.00%
Mexico
Income Taxes
Mexican royalty			$ 32.3	$ 13.7
Mining royalty tax (as a percent)			7.50%
Additional royalty tax over net sales from sales of gold, silver and platinum (as a percent)			0.50%
2016 | Peru
Income Taxes
Income Tax Rate (as a percent )			28.00%
Dividend Tax Rate (as a percent )			6.80%
2017 and later | Peru
Income Taxes
Income Tax Rate (as a percent )			29.50%
Dividend Tax Rate (as a percent )			5.00%