Miscellaneous Other Income (Schedule of Miscellaneous Other Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Component of Operating Income [Abstract]
Royalty Income - Non-Operated Coal									$ 24,722	$ 28,089	$ 19,739
Purchased Coal Sales									19,152	13,161	5,757
Property Easements and Option Income									5,644	2,436	11,281
Rental Income									3,804	14,114	34,789
Interest Income									2,146	2,619	1,166
Contract Buyout									350	9,912	6,288
Other									2,842	2,948	3,100
Miscellaneous Other Income	$ 11,426	$ 10,978	$ 10,369	$ 25,887	$ 20,771	$ 19,713	$ 10,145	$ 22,650	$ 58,660	$ 73,279	$ 82,120