FAIR VALUE MEASUREMENT - Summary of carrying values and fair values of financial instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Trade and other payables [Member]
Financial Liabilities
Carrying value	$ 111.3	$ 101.3
Fair value	111.3	101.3
Long-term debts [Member]
Financial Liabilities
Carrying value	714.5	780.5
Fair value	707.7	652.9
Gold stream obligation [Member]
Financial Liabilities
Carrying value	164.5	182.4
Fair value	164.5	182.4
Performance Share Units [Member]
Financial Liabilities
Carrying value	0.2	0.2
Fair value	0.2	0.2
Restricted share units [Member]
Financial Liabilities
Carrying value	0.5	0.3
Fair value	0.5	0.3
Gold price option contracts [Member]
Financial Liabilities
Carrying value	26.4	4.8
Fair value	26.4	4.8
Cash and cash equivalents
Financial Assets
Carrying value	83.4	103.7
Fair value	83.4	103.7
Trade and other receivables [Member]
Financial Assets
Carrying value	14.5	36.6
Fair value	14.5	36.6
Provisionally priced contracts [Member]
Financial Assets
Carrying value	1.5	(1.6)
Fair value	1.5	(1.6)
Copper price option contracts [Member]
Financial Assets
Carrying value	0.0	0.7
Fair value	0.0	0.7
Gold and copper swap contracts [Member]
Financial Assets
Carrying value	(1.3)	0.9
Fair value	(1.3)	0.9
Investment [Member]
Financial Assets
Carrying value	0.5	0.8
Fair value	0.5	0.8
Proceeds due from income tax refunds at Mesquite [Member]
Financial Assets
Carrying value	9.0	8.5
Fair value	9.0	8.5
Deferred share units [Member]
Financial Liabilities
Carrying value	1.1	0.3
Fair value	$ 1.1	$ 0.3

The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
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