Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		$ 20,500,772	$ 20,924,003
Property, plant and equipment, Net		8,220,031	8,316,188
Exploration and Production [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost	[1]	20,329,930	20,767,772
Property, plant and equipment, Net	[1],[2]	8,120,293	8,214,740
Corporate and Other [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		170,842	156,231
Property, plant and equipment, Net		99,738	101,448
Mineral Rights [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		600,423	595,138
Property, plant and equipment, Net		198,349	188,689
Administrative Assets and Support Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Net		$ 38,670	$ 48,053

[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.