Long-Term Debt (Revolving Credit Facility - Narrative) (Details) (USD $)	9 Months Ended					9 Months Ended								9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Letters Of Credit [Member]		Dec. 31, 2012 Letters Of Credit [Member]		Sep. 30, 2013 Forecasted Production In Current Year Attributable To Proved Developed Producing Reserves And Reserves To Be Acquired [Member]	Sep. 30, 2013 Forecasted Production For The Next Two Years From Total Proved Reserves And Total Proved Reserves To Be Acquired [Member]	Sep. 30, 2013 Forecasted Production For Three Plus Years From Total Proved Reserves And Total Proved Reserves To Be Acquired [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] item	Oct. 31, 2013 Senior Secured Revolving Credit Facility [Member]	Oct. 15, 2013 Senior Secured Revolving Credit Facility [Member]	Jan. 15, 2013 Senior Secured Revolving Credit Facility [Member]	Jan. 15, 2013 Senior Secured Revolving Credit Facility [Member] Letters Of Credit [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Minimum [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Maximum [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Interest Rate Option 1 [Member] Base Rate [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Interest Rate Option 1 [Member] London Interbank Offered Rate (LIBOR) [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Interest Rate Option 1 [Member] Minimum [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Interest Rate Option 1 [Member] Maximum [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Interest Rate Option 2 [Member] Minimum [Member]	Sep. 30, 2013 Senior Secured Revolving Credit Facility [Member] Interest Rate Option 2 [Member] Maximum [Member]
Debt Instrument [Line Items]
Credit Agreement, initiation date									Dec. 22, 2010
Credit Agreement, amount outstanding		$ 23,488,000	[1]	$ 23,488,000	[1]					$ 600,000,000
Credit Agreement, prior maximum borrowing capacity													30,000,000
Credit Agreement, maximum borrowing capacity									1,500,000,000		950,000,000
Credit Agreement, prior current borrowing capacity												730,000,000	20,000,000
Credit Agreement, current borrowing capacity												900,000,000
Credit Agreement, expiration date									Apr. 20, 2017
Credit Agreement, remaining borrowing capacity									$ 276,500,000	$ 326,500,000
Percentage multiplied by stated principal amount of debt issuance to reduce borrowing base									0.25
Percentage of oil and gas properties collateralized by liens														80.00%
Credit Agreement, interest rate description									. ETSWDC is not a subsidiary guarantor under our Credit Agreement. Borrowings bear interest at our option of either (i) the greater of the prime rate of Wells Fargo Bank, National Association, the federal funds effective rate plus 0.50%, or the one-month adjusted LIBOR plus 1.0%, all of which would be subject to a margin that varies from 0.50% to 1.50% per annum according to the borrowing base usage (which is the ratio of outstanding borrowings and letters of credit to the borrowing base then in effect), or (ii) the applicable LIBOR plus a margin that varies from 1.50% to 2.50% per annum according to the borrowing base usage
Credit Agreement, basis spread on variable rate																0.50%	1.00%	0.50%	1.50%	1.50%	2.50%
Credit Agreement, unused portion, commitment fee percentage														0.375%	0.50%
Total debt to EBITDAX ratio to be maintained for Credit Agreement									4.0
Current ratio to be maintained for Credit Agreement									1.0
Number of days after fiscal year end to provide audited financial statements for credit agreement	90 days
Number of days after fiscal quarter end to provide unaudited financial statements for credit agreement	45 days
Percentage of forecasted production prohibiting entering into derivative contracts						90.00%	85.00%	75.00%

[1] These letters of credit relate to a reclamation deposit requirement of $23.4 million and others totaling $0.1 million. Refer to Note 11 - Commitment and Contingencies for details