Exploratory Well Costs (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Exploratory Well Costs [Abstract]
Balance at January 1	$ 61,111,000	$ 99,801,000	$ 118,459,000
Additions Pending the Determination of Proved Reserves	73,332,000	31,271,000	94,090,000
Reclassifications to Proved Properties	(69,462,000)	(29,227,000)	(93,333,000)
Costs Charged to Expense (1)	(17,115,000)	(42,178,000)	(20,267,000)
Foreign Currency Translations	1,250,000	1,444,000	852,000
Balance at December 31	49,116,000	61,111,000	99,801,000
Capitalized exploratory well costs that have been capitalized for a period less than one year	28,319,000	17,009,000	43,408,000
Capitalized exploratory well costs that have been capitalized for a period greater than one year	20,797,000	44,102,000	56,393,000
Number of exploratory wells that have been capitalized for a period greater than one year	1	4	4
Exploratory well costs related to an outside operated, offshore Central North Sea project in the United Kingdom		20,000,000	21,000,000
Exploratory well costs related to an East Irish Sea project in the United Kingdom		9,000,000	9,000,000
Exploratory well costs related to a project in the Sichuan Basin, Sichuan Province, China		9,000,000	20,000,000
Exploratory well costs related to a shale project in British Columbia, Canada		$ 6,000,000	$ 6,000,000