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CWP and Workers Comp - Net Periodic Costs (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Amortization of actuarial gain $ 6,303 $ 5,511
Defined Benefit Plan, Net Periodic Benefit Cost 5,554 5,363
Remeasurement of the Actuarially Determined Long-Term Liabilities   7,713
Workers Compensation [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 1,463 1,904
Interest cost 592 638
Amortization of actuarial gain (153) (101)
Defined Benefit Plan, Administration Expenses 138 0
Post Employment Benefit Plan, State Administrative Fees and Insurance Bond Premiums 799 731
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments 0 0
Defined Benefit Plan, Net Periodic Benefit Cost 2,839 3,172
Coal Workers Pneumoconiosis [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Service cost 1,129 1,204
Interest cost 1,013 1,123
Amortization of actuarial gain (1,908) (1,383)
Defined Benefit Plan, Administration Expenses 151 0
Post Employment Benefit Plan, State Administrative Fees and Insurance Bond Premiums 0 0
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments 0 1,307
Defined Benefit Plan, Net Periodic Benefit Cost $ 385 (363)
Buchanan and Other Assets [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]    
Defined Benefit Plan, Net Periodic Benefit Cost   $ 1,290