Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Details)

v3.5.0.2
Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Asset Retirement Obligation Rollforward      
Estimated Future Cash Undiscounted Amount $ 27,200    
Beginning of year 26,061 $ 23,445 $ 23,445
Change in estimated liability 150   2,101
Accretion expense 399 $ 383 515
End of period $ 26,610   $ 26,061