Other Long-Term Assets and Other Income (Schedule of Other Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other income:
MSHA reimbursements	$ 1,725	$ 1,753
Miscellaneous	1,341	2,209
Other income	$ 3,066	$ 3,962	$ 2,236