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Asset Retirement Obligations (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Asset Retirement Obligation [Abstract]    
Carrying Amount at Beginning of Period $ 752,718 $ 761,898
Liabilities Incurred 38,095 91,822
Liabilities Settled (1) [1] (12,929) (44,805)
Accretion 23,810 33,833
Revisions (13,576) 68,785
Foreign Currency Translations (4,361) (4,178)
Carrying Amount at End of Period 783,757 907,355
Current Portion 11,592 12,528
Noncurrent Portion $ 772,165 $ 894,827
[1] Includes settlements related to asset sales.