Asset Retirement Obligations - Changes in Asset Retirement Obligation Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation
Balance	$ 7,557	$ 7,839
Accretion	562	623	$ 665
Amounts paid	(248)	(213)
Increases and (reductions)	(463)	(692)
Reclassifications	0	0
Balance	7,408	7,557	7,839
Total estimated liability	9,300
Short-Term Portion
Asset Retirement Obligation
Balance	85	296
Accretion	8	36
Amounts paid	(248)	(213)
Increases and (reductions)	5	(134)
Reclassifications	384	100
Balance	234	85	296
Long-Term Portion
Asset Retirement Obligation
Balance	7,472	7,543
Accretion	554	587
Amounts paid	0	0
Increases and (reductions)	(468)	(558)
Reclassifications	(384)	(100)
Balance	$ 7,174	$ 7,472	$ 7,543