Exploratory Well Costs (Details) (USD $)	3 Months Ended
	Mar. 31, 2013
Exploratory Well Costs [Abstract]
Balance at December 31, 2012	$ 49,116,000
Additions Pending the Determination of Proved Reserves	51,797,000
Reclassifications to Proved Properties	0
Costs Charged to Expense (1)	(1,616,000)
Foreign Currency Translations	(1,546,000)
Balance at March 31, 2013	97,751,000
Capitalized exploratory well costs that have been capitalized for a period less than one year	70,633,000
Capitalized exploratory well costs that have been capitalized for a period greater than one year	27,118,000
Number of exploratory wells that have been capitalized for a period greater than one year	2
Exploratory well costs related to an outside operated, offshore Central North Sea project in the United Kingdom	19,000,000
Exploratory well costs related to a project in Central Alberta, Canada	$ 8,000,000