Miscellaneous Other Income (Schedule of Miscellaneous Other Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Component of Operating Income [Abstract]
Royalty Income - Non-Operated Coal									$ 28,089	$ 19,739	$ 15,356
Rental Income									14,114	34,789	36,908
Purchased Coal Sales									13,161	5,757	1,596
Coal Contract Buyout									9,912	6,288	0
Interest Income									2,619	1,166	410
Right of Way Issuance									2,436	11,281	10,827
Other									2,948	3,100	3,096
Miscellaneous Other Income	$ 20,771	$ 19,713	$ 10,145	$ 22,650	$ 32,418	$ 13,569	$ 20,627	$ 15,506	$ 73,279	$ 82,120	$ 68,193