Suspended Exploratory Well Costs (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012 Project	Dec. 31, 2011 Project
Suspended Exploratory Well Costs
Balance at beginning of period	$ 93,388	$ 23,908
Additions to capitalized exploratory well costs pending the determination of proved reserves	150,136	86,996
Reclassifications to wells, facilities and equipment based on determination of proved reserves	(178,423)	(17,516)
Divested wells	(4,980)
Capitalized exploratory well costs charged to expense
Balance at end of period	60,121	93,388
Less exploratory well costs that have been capitalized for a period of one year or less	(39,107)	(83,860)
Capitalized exploratory well costs that have been capitalized for a period greater than one year	$ 21,014	$ 9,528
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year	8	3