INCOME TAXES: (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Income Taxes
Statutory income tax provision			$ 175.5	$ 249.8
Peruvian royalty				2.9
Mexican royalty			13.7	22.3
Peruvian special mining tax			5.0	11.2
Total income tax provision	$ 102.0	$ 157.0	$ 194.2	$ 286.2
Effective income tax rate (as a percent)			32.70%	33.30%
Changes in the entity's uncertain tax positions			$ 0.0
Peru
Income Taxes
Peruvian royalty			$ 0.0		$ 2.9
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			$ 8.1	$ 13.7
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			$ 5.0	11.2
Peru | Minimum
Income Taxes
Mining royalty tax (as a percent)			1.00%
Royalty charge assessed as a percentage of net sales			1.00%
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			$ 13.7	$ 22.3
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%
2015 - 2016
Income Taxes
Income tax rate ( as a percent)			28.00%
Dividend tax rate			6.80%
2017 - 2018
Income Taxes
Income tax rate ( as a percent)			27.00%
Dividend tax rate			8.00%
2019 and later
Income Taxes
Income tax rate ( as a percent)			26.00%
Dividend tax rate			9.30%