INCOME TAXES: - Other (Details) S/ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 PEN (S/)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 PEN (S/)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 PEN (S/)	Dec. 31, 2016 USD ($)	Oct. 31, 2017	Sep. 30, 2017
Income Taxes
Cash, cash equivalents, restricted cash and short-term investments		$ 1,058.4
Estimated one time transition tax on foreign earnings and profits				$ 8,900.0
U.S. federal income tax		0.0
State income tax		0.0
Provisional tax amount				181.1
Reduced tax amount from provisional tax amount		28.0
Finalized transition tax liability		153.1
Deferred tax asset		128.9		72.2
State income tax liability		0.0
Withholding taxes due to the tax treaty between the United States and Mexico		0.0
Withholding tax rate on interest							0.46%	0.58%
Changes in unrecognized tax benefits
Unrecognized tax benefits, opening balance		214.5		304.0		$ 400.0
Gross increases - tax positions in prior period						3.9
Gross decreases - tax positions in prior period				(89.5)
Gross increases - current-period tax positions						23.3
Decreases related to settlements with taxing authorities						(123.2)
Increase (decrease) in unrecognized tax benefits				(89.5)		(96.0)
Unrecognized tax benefits, ending balance		214.5		214.5		304.0
Amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate		$ 214.5		214.5
Period of unrecognized tax benefits are expected to be reversed	12 months	12 months
Accrued interest and penalties included in liability for uncertain tax positions		$ 1.9		1.9
Number of major tax jurisdictions | item		3
Minera Mexico
Income Taxes
Earnings available for dividends to United States				555.5		470.5
Unremitted foreign earnings						45.7
Foreign subsidiaries
Income Taxes
Cash, cash equivalents, restricted cash and short-term investments		$ 97.6
Deferred tax asset		1,900.0
Foreign
Changes in unrecognized tax benefits
Unrecognized tax benefits, ending balance		$ 205.7
Peru
Income Taxes
Royalty charge assessed as a percentage of net sales	1.00%	1.00%
Royalty charges | S/	S/ 32.9		S/ 23.4		S/ 16.8
Provision for royalty tax		$ 9.0		2.5		0.0
Peruvian special mining tax		30.6		$ 23.3		$ 10.8
Changes in unrecognized tax benefits
Unrecognized tax benefits, ending balance		$ 0.0
Peru | Minimum
Income Taxes
Mining royalty tax (as a percent)	1.00%	1.00%
Special mine tax (as a percent)	2.00%	2.00%
Peru | Maximum
Income Taxes
Mining royalty tax (as a percent)	12.00%	12.00%
Special mine tax (as a percent)	8.40%	8.40%
Mexico
Changes in unrecognized tax benefits
Unrecognized tax benefits, ending balance		$ 0.0