Property, Plant and Equipment (Schedule of Property, Plant and Equipment) (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		$ 21,742,558	$ 22,834,571
Property, plant and equipment, Net		9,818,365	13,331,047	[1]
Exploration and Production Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost	[2]	21,607,962	22,731,220
Property, plant and equipment, Net	[2],[3]	9,723,222	13,277,985
Corporate and Other Items [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		134,596	103,351
Property, plant and equipment, Net		95,143	53,062
Mineral Rights [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Cost		1,075,040	924,253
Property, plant and equipment, Net		612,518	410,482
Administrative Assets and Support Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, Net		$ 50,924	$ 58,334

[1] Reclassified to conform to current presentation.

[2] Includes mineral rights as follows: $1,075,040 612,518 924,253 410,482

[3] Includes $50, 924 in 2015 and $58,334 in 2014 related to administrative assets and support equipment.