XML 53 R43.htm IDEA: XBRL DOCUMENT v3.19.1
Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Changes in asset retirement obligations    
Asset retirement obligations at beginning of period $ 8,087 $ 15,506
Liabilities incurred   113
Liabilities settled   (1)
Liabilities eliminated through asset sales   (62)
Current period accretion expense 157 297
Asset retirement obligations at end of period $ 8,244 $ 15,853