Debt (Narrative) (Details) (USD $)	3 Months Ended			3 Months Ended				6 Months Ended	3 Months Ended			3 Months Ended					3 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Jun. 30, 2012 Maximum [Member]	Jun. 30, 2012 Maximum [Member] Fiscal Quarter Ending June 30, 2012 [Member]	Sep. 30, 2012 Maximum [Member] Fiscal Quarters Ending September 30, 2012 [Member]	Dec. 31, 2012 Maximum [Member] Fiscal Quarters Ending December 31, 2012 [Member]	Jun. 30, 2012 Maximum [Member] Fiscal Quarters Ending March 31, 2013 And Thereafter [Member]	Jun. 30, 2012 Minimum [Member]	Jun. 30, 2012 Senior Secured Revolving Credit Facility [Member]	Dec. 31, 2011 Senior Secured Revolving Credit Facility [Member]	Jun. 30, 2012 U.K. Huntington Field Development Project Credit Facility [Member]	Dec. 31, 2011 U.K. Huntington Field Development Project Credit Facility [Member]	Jun. 30, 2012 Term Loan Facility [Member]	Jun. 30, 2012 Contingent Cost Overrun Term Loan Facility [Member]	Jun. 30, 2012 Post-Completion Credit Facility [Member]	Jun. 30, 2012 Senior Notes [Member]	Dec. 31, 2014 Scenario, Forecast [Member] U.K. Huntington Field Development Project Credit Facility [Member]	Jun. 30, 2014 Scenario, Forecast [Member] U.K. Huntington Field Development Project Credit Facility [Member]	Dec. 31, 2013 Scenario, Forecast [Member] U.K. Huntington Field Development Project Credit Facility [Member]	Jun. 30, 2013 Scenario, Forecast [Member] U.K. Huntington Field Development Project Credit Facility [Member]	Jun. 30, 2012 Convertible Senior Notes [Member]	Dec. 31, 2011 Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Aggregate principal amount																	$ 200,000,000
Debt instrument issuance date																	November 17, 2011
Convertible Debt																						73,750,000	73,750,000
Debt instrument interest rate																						4.375%
Convertible Debt, Percent of Principal Amount Plus Accrued and Unpaid Interest to Repurchase	100.00%
Line of credit facility, maximum borrowing capacity	750,000,000													55,000,000	6,500,000	22,500,000
Line of credit facility, expiration date	January 27, 2016
Line of credit facility initial borrowing base	350,000,000
Line of credit facility current borrowing base	325,000,000
Ratio of total debt to EBITDA	3.40				4.75	4.25	4.25	4.00
Current ratio	1.13								1.00
Ratio of senior debt to EBITDA	0.48			2.50
Ratio of EBITDA to interest expense	4.63								2.50
Line of credit facility amount outstanding										130,000,000	47,000,000	37,000,000	17,813,000
Current maturities of long-term debt	(16,650,000)	0	0									(16,650,000)
Revolving Credit Facility, weighted average interest rate	2.78%											3.97%
Letters of credit outstanding amount										1,000,000
Repayment schedule reflected as a percentage																		15.00%	20.00%	20.00%	45.00%
Current total availability of term loan facility												55,000,000
Current total availability of cost overrun facility												$ 6,500,000