Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Cost	$ 19,724,343	$ 19,303,389
Net	8,269,038	9,969,743
Exploration and Production
Property, Plant and Equipment [Line Items]
Cost	19,583,682	19,096,323
Net	8,232,191	9,875,727
Corporate and other
Property, Plant and Equipment [Line Items]
Cost	140,661	207,066
Net	36,847	94,016
Mineral Rights
Property, Plant and Equipment [Line Items]
Cost	649,704	508,526
Net	530,194	121,163
Administrative Assets and Support Equipment
Property, Plant and Equipment [Line Items]
Net	$ 22,940	$ 24,698