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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Asset Retirement Obligations [Abstract]    
Carrying Amount at Beginning of Period $ 761,898 $ 665,944
Liabilities Incurred 37,453 12,592
Liabilities Settled (1) (6,850) [1] (24,831) [1]
Accretion 13,751 9,660
Revisions 0 67
Foreign Currency Translations (6,636) (5,201)
Carrying Amount at End of Period 799,616 658,231
Current Portion 37,308 19,186
Noncurrent Portion $ 762,308 $ 639,045
[1] Includes settlements related to asset sales.