Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		$ 22,823,315	$ 20,500,772
Property, plant and equipment, Net		9,757,564	8,220,031
Exploration and Production [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost	[1]	22,629,844	20,329,930
Property, plant and equipment, Net	[1],[2]	9,654,945	8,120,293
Corporate and Other [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		193,471	170,842
Property, plant and equipment, Net		102,619	99,738
Mineral Rights [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		512,025	600,423
Property, plant and equipment, Net		144,912	198,349
Administrative Assets and Support Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Net		$ 32,071	$ 38,670

[1] Includes mineral rights as follows: $600,423 198,349 595,138 188,689

[2] Includes $38,670 in 2017 and $48,053 in 2016 related to administrative assets and support equipment.