LINES OF CREDIT AND LONG-TERM DEBT - Additional Information (Detail) (USD $)	0 Months Ended	6 Months Ended			6 Months Ended	0 Months Ended			3 Months Ended							0 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 24, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Senior Notes	Jun. 25, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition	Dec. 24, 2013 Canadian Acquisition	Jun. 30, 2014 Canadian Acquisition	Apr. 28, 2014 Canadian Acquisition Senior Notes	Jun. 30, 2014 Kemmerer Mine	Apr. 28, 2014 Revolving Credit Facility	Apr. 28, 2014 Revolving Credit Facility Canadian Acquisition	Apr. 28, 2014 Subfacility for Letters of Credit	Jun. 30, 2014 Add On Notes - Senior Notes due 2018 Senior Notes	Feb. 07, 2014 Senior Notes due 2018 Senior Notes	Jun. 30, 2014 Senior Notes due 2018 Senior Notes	Jun. 30, 2014 Senior Notes due 2018 Senior Notes	Dec. 31, 2013 Senior Notes due 2018 Senior Notes
Debt Instrument [Line Items]
Aggregate principal amount					$ 675,500,000					$ 425,000,000					$ 425,000,000	$ 425,000,000
Stated interest rate (percent)					10.75%					10.75%					10.75%	10.75%	10.75%	10.75%	10.75%
Debt instrument price as percentage of face value																106.875%
Proceeds from issuance of debt																454,200,000
Debt premium																29,200,000
Cash paid		322,637,000	0			39,800,000	282,800,000	283,000,000	322,600,000
Transaction costs for acquisition and note offering	24,000,000
Maximum borrowing capacity												60,000,000	60,000,000	60,000,000
Debt issuance costs capitalized		15,400,000
Debt repurchased					1,000,000
Principal amount of senior notes repurchased					1,000,000
Loss on restructuring of debt					(100,000)
Excess cash flow																			29,500,000
Excess cash flow offered for repurchase of debt																	22,100,000
Current installments of long-term debt		42,401,000		44,343,000															22,100,000
Repayments of debt																		1,000,000
Redemption price as percentage of face value of debt																		100.00%
Long-term debt, less current installments		790,643,000		295,494,000													21,100,000	21,100,000
Capital lease obligations											$ 12,900,000