Miscellaneous Other Income (Schedule of Miscellaneous Other Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Component of Operating Income [Abstract]
Royalty Income - Non-Operated Coal									$ 22,208	$ 24,722	$ 28,089
Purchased Coal Sales									12,385	19,152	13,161
Contract Buyout									9,959	350	9,912
Interest Income									2,937	2,146	2,619
Rental Income									2,517	3,804	14,114
Property Easements and Option Income									1,631	5,644	2,436
Other									1,712	2,842	2,948
Miscellaneous Other Income	$ 16,675	$ 11,188	$ 12,194	$ 13,292	$ 11,426	$ 10,978	$ 10,369	$ 25,887	$ 53,349	$ 58,660	$ 73,279