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Property, Plant and Equipment - Capitalized Costs of Unproved Properties Excluded from Amortization (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Abstract]        
Property acquisition, cumulative $ 362,803     $ 80,639
Property acquisition 197,849 $ 70,304 $ 14,011  
Exploration, cumulative 34,988     339
Exploration 10,698 6,263 17,688  
Total costs incurred, cumulative 397,791     $ 80,978
Total costs incurred $ 208,547 $ 76,567 $ 31,699