Summary of Significant Accounting Policies (Changes to ARO) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies [Abstract]
Balance beginning of year	$ 13,260	$ 12,231
Accretion	861	1,029	$ 498
Revision	(112)
Other payment	(203)
Balance end of year	$ 13,260	$ 12,231	$ 12,231	$ 13,806	$ 13,260
Less current portion				(300)	(145)
Long-term balance, end of year				$ 13,506	$ 13,115