Coal Workers' Pneumoconiosis and Workers' Compensation (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
CWP
Defined Benefit Plan Disclosure [Line Items]
Increase in plan liability	$ 25,700	$ 41,700

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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