Debt (Narrative) (Details)	3 Months Ended		3 Months Ended									0 Months Ended		3 Months Ended					3 Months Ended
	Mar. 31, 2012 USD ($)	Apr. 30, 2012 USD ($)	Mar. 31, 2012 Maximum [Member]	Mar. 31, 2012 Maximum [Member] Fiscal Quarter Ending March 31, 2012 [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]	Mar. 31, 2013 Maximum [Member] Fiscal Quarters Ending March 31, 2013 And Thereafter [Member]	Mar. 31, 2012 Minimum [Member]	Mar. 31, 2012 Senior Secured Revolving Credit Facility [Member] USD ($)	Dec. 31, 2011 Senior Secured Revolving Credit Facility [Member] USD ($)	Mar. 31, 2012 Senior Secured Multicurrency Credit Facility [Member] USD ($)	Dec. 31, 2011 Senior Secured Multicurrency Credit Facility [Member] USD ($)	Mar. 31, 2012 U.K. Huntington Field Development Project Credit Facility [Member] USD ($)	Mar. 31, 2012 U.K. Huntington Field Development Project Credit Facility [Member] GBP (£)	Mar. 31, 2012 Term Loan Facility [Member] USD ($)	Mar. 31, 2012 Contingent Cost Overrun Term Loan Facility [Member] USD ($)	Mar. 31, 2012 Post-Completion Credit Facility [Member] USD ($)	Mar. 31, 2012 Senior Notes [Member] USD ($)	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]
Debt Instrument [Line Items]
Debt instrument issuance date																			November 17, 2011
Aggregate principal amount																			$ 200,000,000
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 adjusted borrowing base	340,000,000
Line of credit facility current borrowing base		325,000,000
Repayment schedule reflected as a percentage																				15.00%	20.00%	20.00%	45.00%
Ratio of total debt to EBITDA	4.08			4.75	4.75	4.25	4.25	4.00
Current ratio	1.00								1.00
Ratio of senior debt to EBITDA	0.76		2.50
Ratio of EBITDA to interest expense	4.91								2.50
Line of credit facility amount outstanding										177,000,000	47,000,000	28,029,000	17,813,000	28,000,000	17,500,000
Revolving Credit Facility, weighted average interest rate	3.08%											4.58%
Letters of credit outstanding amount										1,000,000
Current total availability of term loan facility												55,000,000
Current total availability of cost overrun facility												6,500,000
Transaction loss recorded in other income														$ 900,000