Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation Rollforward
Beginning of year	$ 26,061	$ 23,445
Change in estimated liability	(534)	2,101
Accretion expense	534	515
End of year	$ 26,061	$ 26,061