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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Asset Retirement Obligation [Abstract]    
Carrying Amount at January 1 $ 946,848 $ 912,926
Liabilities Incurred 10,279 7,418
Liabilities Settled (1) [1] 50 (11,197)
Accretion 8,868 8,525
Revisions (291) 3,646
Foreign Currency Translations 245 837
Carrying Amount at March 31 965,999 922,155
Current Portion 19,177 19,450
Noncurrent Portion $ 946,822 $ 902,705
[1] Includes settlements related to asset sales.