Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Weighted-Average Assumptions Used) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (as a percent)	3.75%	4.40%	4.60%
Discount rate (as a percent)	4.40%	4.60%	4.21%
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (as a percent)	3.57%	4.05%	4.26%
Discount rate (as a percent)	4.05%	4.26%	3.84%