Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Weighted-Average Assumptions Used) (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (as a percent)	3.41%	4.42%	3.75%
Discount rate (as a percent)	4.42%	3.75%	4.40%
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (as a percent)	3.25%	4.26%	3.57%
Discount rate (as a percent)	4.26%	3.57%	4.05%