Asset Retirement Obligations (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Asset Retirement Obligation Rollforward
Estimated Future Cash Undiscounted Amount	$ 27,300
Beginning of year	26,061	$ 26,061
Change in estimated liability	815	(534)
Accretion expense	266	534
End of year	$ 27,142	$ 26,061