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STOCK BASED COMPENSATION - Stock Options (Details) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2015 USD ($) CAD / shares $ / shares shares | Dec. 31, 2015 USD ($) $ / shares shares | Dec. 31, 2014 USD ($) $ / shares shares | Dec. 31, 2013 USD ($) $ / shares shares | Dec. 31, 2012 $ / shares shares
STOCK BASED COMPENSATION |  |  |  |  |
Stock options granted (in shares) |  | 2,733,000 | 1,731,000 | 1,728,000 |
Exercise price of options granted (in dollars per share) | $ / shares |  | $ 1.03 | $ 2.90 | $ 2.26 |
Exercisable period of options |  | 3 years 8 months 12 days |  |  |
Number of shares of common stock reserved for issuance | 17,500,000 | 17,500,000 |  |  |
Shares |  |  |  |  |
Balance at the beginning of the period (in shares) |  | 4,651,000 | 4,841,000 | 3,561,000 |
Granted (in shares) |  | 2,733,000 | 1,731,000 | 1,728,000 |
Exercised (in shares) |  | (272,000) | (1,499,000) | (48,000) |
Forfeited (in shares) |  | (158,000) | (422,000) | (400,000) |
Balance at the end of the period (in shares) |  | 6,954,000 | 4,651,000 | 4,841,000 | 3,561,000
Exercisable at the end of the period (in shares) | 2,772,000 | 2,772,000 |  |  |
Weighted Average Exercise Price |  |  |  |  |
Balance at the beginning of the period (in dollars per share) | $ / shares |  | $ 3.35 | $ 2.96 | $ 3.47 |
Granted (in dollars per share) | $ / shares |  | 1.03 | 2.90 | 2.26 |
Exercised (in dollars per share) | $ / shares |  | 1.29 | 1.29 | 1.97 |
Forfeited (in dollars per share) | $ / shares |  | 2.82 | 4.39 | 4.57 |
Expired (in dollars per share) | $ / shares |  | 2.12 |  |  |
Balance at the end of the period (in dollars per share) | $ / shares |  | 2.49 | $ 3.35 | $ 2.96 | $ 3.47
Exercisable (in dollars per share) | $ / shares | $ 3.80 | $ 3.80 |  |  |
Weighted Average Remaining Contractual Life |  |  |  |  |
Outstanding at the end of the period |  | 4 years 1 month 6 days | 4 years 4 months 24 days | 5 years 2 months 12 days | 6 years 7 months 6 days
Exercisable period of options |  | 3 years 8 months 12 days |  |  |
Intrinsic Value |  |  |  |  |
Exercised (in dollars) | $ |  |  | $ 438 | $ 47 |
Outstanding at the end of the period (in dollars) | $ | $ 98 | $ 98 |  |  |
Exercisable at the end of the period (in dollars) | $ | 8 | $ 8 |  |  |
Granted (in shares) |  | 2,733,000 | 1,731,000 | 1,728,000 |
Principal assumptions used in applying the Black-Scholes option pricing model for the awards |  |  |  |  |
Risk-free interest rate, low end of range (as a percent) |  | 1.10% |  | 0.50% |
Risk-free interest rate, high end of range (as a percent) |  | 1.79% |  | 0.86% |
Dividend yield (as a percent) |  |  | 0.00% | 0.00% |
Volatility factor of the expected market price of common stock, low end of range (as a percent) |  | 73.00% |  | 66.00% |
Volatility factor of the expected market price of common stock, high end of range (as a percent) |  | 74.00% |  | 69.00% |
Risk-free interest rate (as a percent) |  | 1.01% |  |  |
Volatility factor of the expected market price of common stock (as a percent) |  |  | 70.00% |  |
Weighted-average expected life of option |  |  | 3 years 6 months | 3 years 6 months |
Weighted-average grant date fair value (in dollars per share) | $ / shares |  | $ 0.49 | $ 1.45 | $ 1.02 |
Stock option expense | $ |  | $ 1,300 | $ 1,300 | $ 1,400 |
Additional disclosures |  |  |  |  |
Unrecognized compensation expense on non-vested stock options (in dollars) | $ | $ 1,600 | $ 1,600 | $ 1,800 |  |
Non-vested stock options outstanding (in shares) |  | 4,200,000 | 2,700,000 |  |
Weighted-average period of recognition |  | 1 year 7 months 6 days | 1 year 7 months 6 days | 1 year 4 months 24 days |
Minimum |  |  |  |  |
Principal assumptions used in applying the Black-Scholes option pricing model for the awards |  |  |  |  |
Dividend yield (as a percent) |  | 0.00% |  |  |
Weighted-average expected life of option |  | 3 years 6 months |  |  |
Maximum |  |  |  |  |
STOCK BASED COMPENSATION |  |  |  |  |
Maximum number of shares that may be subject to grants of options to an individual in a calendar year |  | 1,000,000 |  |  |
Principal assumptions used in applying the Black-Scholes option pricing model for the awards |  |  |  |  |
Dividend yield (as a percent) |  | 1.15% |  |  |
Minera Andes |  |  |  |  |
Shares |  |  |  |  |
Exercised (in shares) |  | (198,000) |  |  |
Weighted Average Exercise Price |  |  |  |  |
Exercised (in dollars per share) | CAD / shares | $ 1.96 |  |  |  |
Certain employees and directors |  |  |  |  |
STOCK BASED COMPENSATION |  |  |  |  |
Stock options granted (in shares) |  | 2,700,000 | 1,700,000 | 1,700,000 |
Exercise price of options granted (in dollars per share) | $ / shares |  | $ 1.03 | $ 2.90 | $ 2.26 |
Shares |  |  |  |  |
Granted (in shares) |  | 2,700,000 | 1,700,000 | 1,700,000 |
Weighted Average Exercise Price |  |  |  |  |
Granted (in dollars per share) | $ / shares |  | $ 1.03 | $ 2.90 | $ 2.26 |
Intrinsic Value |  |  |  |  |
Granted (in shares) |  | 2,700,000 | 1,700,000 | 1,700,000 |
Vesting period of options |  | 3 years |  |  |
Exercise period of options |  | 5 years |  |  |

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- Definition The risk-free interest rate assumption that is used in valuing an option on its own shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(iv) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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