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ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS - Amortized Cost, Gross Unrealized Holding Gains and Losses and Fair Value of Held-to-Maturity Securities (Detail) (Reclamation Deposits [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclamation Deposits [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 25,688	$ 11,396	$ 19,832
Gross unrealized holding gains	826	764	1,190
Gross unrealized holding losses	(113)	(74)	(9)
Fair value	$ 26,401	$ 12,086	$ 21,013

This item represents investments in debt securities which are categorized as held-to-maturity and that have scheduled maturities more than one year from the balance sheet date or operating cycle, if longer, and that are pledged to one or more secured parties who have the right to buy, sell, or re-pledge the collateral.

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