Debt (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	May 31, 2012
Line of Credit Facility [Line Items]
Long term debt	$ 101,509,000	$ 110,507,000
Equipment financing credit facility | Line of Credit
Line of Credit Facility [Line Items]
Long term debt	9,881,000	11,232,000
Equipment financing credit facility | Line of Credit | Caterpillar Financial Services Corporation
Line of Credit Facility [Line Items]
Credit facility borrowing capacity	35,000,000
Debt instrument basis spread on variable rate	2.38%
Credit available to draw down	18,700,000	16,800,000
Average interest rate on outstanding loans	6.70%	6.60%
4.00% Convertible Debentures | Convertible Debt
Line of Credit Facility [Line Items]
Debt instrument interest rate	4.00%
Debt amount exchanged with private holders			74,500,000
5.00% Convertible Debentures | Convertible Debt
Line of Credit Facility [Line Items]
Debt instrument interest rate	5.00%
Long term debt	91,628,000	99,275,000
Oustanding amount of convertible debenture			$ 77,500,000
Conversion price per share (dollars per share)	$ 1.65
Share issue value (percent)	95.00%
Convertible debt conversion rate per $1000 principal	606.0606
Convertible debt, conversion price above closing stock price (percent)	25.00%
Minimum percentage of oustanding debentures required to have accepted mandatory offer to purchase	90.00%
New Mining Equipment [Member] | Equipment financing credit facility | Line of Credit | Caterpillar Financial Services Corporation
Line of Credit Facility [Line Items]
Debt Instrument, Repayment Period	5 years
Used Mining Equipment [Member] | Equipment financing credit facility | Line of Credit | Caterpillar Financial Services Corporation
Line of Credit Facility [Line Items]
Debt Instrument, Repayment Period	2 years