Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		$ 20,924,003	$ 21,742,558
Property, plant and equipment, Net		8,316,188	9,818,365	[1]
Exploration and Production [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost	[2]	20,767,772	21,607,962
Property, plant and equipment, Net	[2],[3]	8,214,740	9,723,222
Corporate and Other [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		156,231	134,596
Property, plant and equipment, Net		101,448	95,143
Mineral Rights [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		595,138	1,075,040
Property, plant and equipment, Net		188,689	612,518
Administrative Assets and Support Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Net		$ 48,053	$ 50,924

[1] Reclassified to conform to current presentation. See Note B for additional information.

[2] Includes mineral rights as follows: $595,138 188,689 1,075,040 612,518

[3] Includes $48,053 in 2016 and $50,924 in 2015 related to administrative assets and support equipment.