Long-Term Debt (Revolving Credit Facility - Narrative) (Details) (USD $)	1 Months Ended	12 Months Ended
	Jul. 31, 2012	Dec. 31, 2012 item	Jan. 15, 2013	Apr. 30, 2012
Letters Of Credit [Member]
Debt Instrument [Line Items]
Credit Agreement, amount outstanding		$ 23,488,000
Senior Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit Agreement, initiation date		Dec. 22, 2010
Credit Agreement, amount outstanding		470,000,000
Credit Agreement, prior maximum borrowing capacity				750,000,000
Credit Agreement, maximum borrowing capacity				1,500,000,000
Credit Agreement, prior current borrowing capacity	730,000,000		730,000,000	630,000,000
Credit Agreement, current borrowing capacity	655,000,000		900,000,000	730,000,000
Credit Agreement, prior expiration date		Dec. 22, 2015
Credit Agreement, expiration date		Apr. 20, 2017
Credit Agreement, reduction in borrowing capacity	75,000,000
Credit Agreement, write-off of deferred loan costs	700,000
Credit Agreement, remaining borrowing capacity		236,500,000
Credit Agreement, interest rate during period including the impact of hedges		4.60%
Credit Agreement, interest rate at end of period excluding potential impact of hedges		2.80%
Percentage multiplied by stated principal amount of debt issuance to reduce borrowing base		0.25
Credit Agreement, interest rate description		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.
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
Senior Secured Revolving Credit Facility [Member] | Letters Of Credit [Member]
Debt Instrument [Line Items]
Credit Agreement, amount outstanding		23,500,000
Credit Agreement, prior current borrowing capacity			20,000,000
Credit Agreement, current borrowing capacity			$ 30,000,000
Senior Secured Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Percentage of oil and gas properties collateralized by liens		80.00%
Credit Agreement, unused portion, commitment fee percentage		0.375%
Senior Secured Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Credit Agreement, unused portion, commitment fee percentage		0.50%
Senior Secured Revolving Credit Facility [Member] | Interest Rate Option 1 [Member]
Debt Instrument [Line Items]
Interest percentage added to Federal Funds Rate		0.50%
Interest percentage added to LIBOR Rate		1.00%
Senior Secured Revolving Credit Facility [Member] | Interest Rate Option 1 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Credit Agreement, basis spread on variable rate		0.50%
Senior Secured Revolving Credit Facility [Member] | Interest Rate Option 1 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Credit Agreement, basis spread on variable rate		1.50%
Senior Secured Revolving Credit Facility [Member] | Interest Rate Option 2 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Credit Agreement, basis spread on variable rate		1.50%
Senior Secured Revolving Credit Facility [Member] | Interest Rate Option 2 [Member] | Maximum [Member]
Debt Instrument [Line Items]
Credit Agreement, basis spread on variable rate		2.50%
Senior Secured Revolving Credit Facility [Member] | Forecasted Production In Current Year Attributable To Proved Developed Producing Reserves And Reserves To Be Acquired [Member]
Debt Instrument [Line Items]
Percentage of forecasted production prohibiting entering into derivative contracts		90.00%
Senior Secured Revolving Credit Facility [Member] | Forecasted Production For The Next Two Years From Total Proved Reserves And Total Proved Reserves To Be Acquired [Member]
Debt Instrument [Line Items]
Percentage of forecasted production prohibiting entering into derivative contracts		85.00%
Senior Secured Revolving Credit Facility [Member] | Forecasted Production For Three Plus Years From Total Proved Reserves And Total Proved Reserves To Be Acquired [Member]
Debt Instrument [Line Items]
Percentage of forecasted production prohibiting entering into derivative contracts		75.00%