POSTRETIREMENT MEDICAL BENEFITS Pneumoconiosis (Black Lung) Benefit Obligation (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Pneumoconiosis Benefit Obligation [Abstract]
Net benefit obligation at end of year	$ 211,100	$ 158,700
Pneumoconiosis (Black Lung) Benefits [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Net benefit obligation at end of year	14,007	14,143
Plan assets at fair value, primarily government-backed securities	2,755	5,468
Unfunded status at end of year	11,252	8,675
Rate used to determine benefit obligations as of the end of the year shown	3.40%	4.00%
Pneumoconiosis (Black Lung) Benefits [Member] \| Quoted prices in active markets for identical assets - Level 1 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	2,755	5,283
Pneumoconiosis (Black Lung) Benefits [Member] \| Significant other observable inputs - Level 2 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	0	185
Pneumoconiosis (Black Lung) Benefits [Member] \| US Treasury Securities [Member] \| Quoted prices in active markets for identical assets - Level 1 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	2,615	5,280
Pneumoconiosis (Black Lung) Benefits [Member] \| US Treasury Securities [Member] \| Significant other observable inputs - Level 2 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	0	0
Pneumoconiosis (Black Lung) Benefits [Member] \| Collateralized Mortgage Backed Securities [Member] \| Quoted prices in active markets for identical assets - Level 1 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets		0
Pneumoconiosis (Black Lung) Benefits [Member] \| Collateralized Mortgage Backed Securities [Member] \| Significant other observable inputs - Level 2 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets		185
Pneumoconiosis (Black Lung) Benefits [Member] \| Cash and cash equivalents [Member] \| Quoted prices in active markets for identical assets - Level 1 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	140	3
Pneumoconiosis (Black Lung) Benefits [Member] \| Cash and cash equivalents [Member] \| Significant other observable inputs - Level 2 [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	0	0
Terminated Employees [Member] \| Pneumoconiosis (Black Lung) Benefits [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Net benefit obligation at end of year	953	876
Existing Claimants [Member] \| Pneumoconiosis (Black Lung) Benefits [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Net benefit obligation at end of year	13,054	13,267
Fair Value [Member] \| Pneumoconiosis (Black Lung) Benefits [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	2,755	5,468
Fair Value [Member] \| Pneumoconiosis (Black Lung) Benefits [Member] \| US Treasury Securities [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	2,615	5,280
Fair Value [Member] \| Pneumoconiosis (Black Lung) Benefits [Member] \| Collateralized Mortgage Backed Securities [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets		185
Fair Value [Member] \| Pneumoconiosis (Black Lung) Benefits [Member] \| Cash and cash equivalents [Member]
Pneumoconiosis Benefit Obligation [Abstract]
Trust assets	$ 140	$ 3

This represents the entire assets recognized in the balance sheet that are associated with the defined benefit plans.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

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- Definition

The interest rate used to adjust for the time value of money for the plan.

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- Definition

1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

+ References

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

-Publisher FASB

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

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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