ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Changes in Company's Asset Retirement Obligations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]
Asset retirement obligations, beginning of period (including current portion)	$ 452,745us-gaap_AssetRetirementObligation	$ 279,864us-gaap_AssetRetirementObligation
Accretion	9,701	5,836
Liabilities settled	(8,195)	(4,437)
Changes due to amount and timing of reclamation	336	0
Asset retirement obligations acquired	3,769	0
Changes due to foreign currency translation	(10,954)	0
Asset retirement obligations, end of period	447,402us-gaap_AssetRetirementObligation	281,263us-gaap_AssetRetirementObligation
Less current portion	(48,024)	(22,227)	(43,289)
Asset retirement obligations, less current portion	$ 399,378	$ 259,036	$ 409,456

ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Changes in Company's Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Dec. 31, 2014

Asset Retirement Obligation Disclosure [Abstract]

Asset retirement obligations, beginning of period (including current portion)

$ 452,745us-gaap_AssetRetirementObligation

$ 279,864us-gaap_AssetRetirementObligation

Accretion

9,701us-gaap_AssetRetirementObligationAccretionExpense

5,836us-gaap_AssetRetirementObligationAccretionExpense

Liabilities settled

(8,195)us-gaap_AssetRetirementObligationLiabilitiesSettled

(4,437)us-gaap_AssetRetirementObligationLiabilitiesSettled

Changes due to amount and timing of reclamation

336us-gaap_AssetRetirementObligationRevisionOfEstimate

0us-gaap_AssetRetirementObligationRevisionOfEstimate

Asset retirement obligations acquired

3,769us-gaap_AssetRetirementObligationLiabilitiesIncurred

0us-gaap_AssetRetirementObligationLiabilitiesIncurred

Changes due to foreign currency translation

(10,954)us-gaap_AssetRetirementObligationForeignCurrencyTranslation

0us-gaap_AssetRetirementObligationForeignCurrencyTranslation

Asset retirement obligations, end of period

447,402us-gaap_AssetRetirementObligation

281,263us-gaap_AssetRetirementObligation

Less current portion

(48,024)us-gaap_AssetRetirementObligationCurrent

(22,227)us-gaap_AssetRetirementObligationCurrent

(43,289)us-gaap_AssetRetirementObligationCurrent

Asset retirement obligations, less current portion

$ 399,378us-gaap_AssetRetirementObligationsNoncurrent

$ 259,036us-gaap_AssetRetirementObligationsNoncurrent

$ 409,456us-gaap_AssetRetirementObligationsNoncurrent

- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

+ Details

Name:	us-gaap_AssetRetirementObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

+ Details

Name:	us-gaap_AssetRetirementObligationAccretionExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_AssetRetirementObligationDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the Increase or Decrease in the amount of asset retirement obligations for the period associated with foreign currency translation (to the extent that the entity accounts for the foreign currency effects as a separate component of the change in the liability).

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationForeignCurrencyTranslation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of asset retirement obligations incurred during the period.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationLiabilitiesIncurred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationLiabilitiesSettled
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationRevisionOfEstimate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant