Property, Plant and Equipment (Schedule Cost and Net of Property, Plant and Equipment) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Property Plant And Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 22,834,571		$ 22,021,294
Property, plant and equipment, net	13,331,047		13,481,055
Exploration and Production Equipment [Member]
Property Plant And Equipment [Line Items]
Property, Plant and Equipment, Gross	22,731,220	[1]	21,932,119	[1]
Property, plant and equipment, net	13,277,985	[1],[2]	13,433,468	[1],[2]
Corporate and Other [Member]
Property Plant And Equipment [Line Items]
Property, Plant and Equipment, Gross	103,351		89,175
Property, plant and equipment, net	53,062		47,587
Mineral Rights [Member]
Property Plant And Equipment [Line Items]
Property, Plant and Equipment, Gross	924,253		1,007,920
Property, plant and equipment, net	410,482		489,578
Administrative Assets And Support Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment, net	$ 58,334		$ 48,691

[1] Includes mineral rights as follows:$924,253410,4821,007,920489,578

[2] Includes $58,334 in 2014 and $48,691 in 2013 related to administrative assets and support equipment.