Supplemental Oil and Gas Disclosures (Details Textuals)	12 Months Ended
	Dec. 31, 2011 MMBoe MMBbls	Dec. 31, 2010 MBoe	Dec. 31, 2009 MBoe
Reserve Quantities [Line Items]
Purchases of minerals in-place	80	593,051,000	20,993,000
Extensions, discoveries and other additions	342	245,108,000	194,903,000
Supplemental oil and gas disclosures (Textual) [Abstract]
Anticipated Timing of Inclusion of Costs in Amortization Calculation	five to ten years
Percentage of estimated proved developed reserves classified as proved not producing	17.00%
Total estimated future net cash flows before income tax expense discounted at 10 percent per annum	52.3	41.6
Sales of properties	54	(73,000)
Estimated future net cash flow before income tax expenses	10.00%
United States [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	64,000,000
Canada [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	107,000,000
Egypt [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	58,000,000
Australia [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	41,000,000
Argentina and North Sea [ Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	19,000,000
Mobil North Sea [Member]
Reserve Quantities [Line Items]
Purchases of minerals in-place	57
Other Acquisitions [Member]
Reserve Quantities [Line Items]
Purchases of minerals in-place	23
Permian Basin [Member] | United States [Member]
Reserve Quantities [Line Items]
Extensions, discoveries and other additions	54,000,000