Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Expected Benefit Payments) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	$ 12,972
2019	10,065
2020	8,841
2021	8,203
2022	8,024
Year 2023-2027	42,525
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	14,390
2019	14,120
2020	14,110
2021	14,035
2022	14,146
Year 2023-2027	53,187
Actuarial Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	11,880
2019	11,547
2020	11,473
2021	11,332
2022	11,375
Year 2023-2027	38,259
Other Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	2,510
2019	2,573
2020	2,637
2021	2,703
2022	2,771
Year 2023-2027	$ 14,928

Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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