Indebtedness (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended			12 Months Ended					12 Months Ended					0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2012 Senior Credit Facility [Member]	Oct. 31, 2012 Senior Credit Facility [Member]	Oct. 30, 2012 Senior Credit Facility [Member]	May 01, 2012 Senior Credit Facility [Member]	Feb. 14, 2011 Senior Credit Facility [Member]	Dec. 31, 2011 Terminated Credit Facility [Member]	Dec. 31, 2012 Minimum [Member] Senior Credit Facility [Member]	Dec. 31, 2012 Maximum [Member] Senior Credit Facility [Member]	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Oct. 25, 2012 2012 Notes [Member]	Dec. 31, 2012 2012 Notes [Member]	Feb. 14, 2011 2011 Notes [Member]	Dec. 31, 2012 2011 Notes [Member]	Dec. 31, 2012 8.25% Notes [Member]	Dec. 31, 2012 8.25% Notes [Member] Minimum [Member]	Dec. 31, 2012 8.25% Notes [Member] Maximum [Member]	Feb. 14, 2012 ASOP Acquisition [Member]	Feb. 14, 2011 ASOP Acquisition [Member]
Debt Instrument [Line Items]
Senior Notes, face amount															$ 300.0	$ 300.0	$ 210.0	$ 210,000,000				$ 210,000,000
Senior Notes, stated percentage																8.25%		8.25%
Maturity date					Oct. 31, 2016										Feb. 15, 2018	Feb. 15, 2018		Feb. 15, 2018
Net proceeds from offering of Notes																289,500,000		202,000,000
Loss on early extinguishment of debt	(2,400,000)	2,377,000	5,627,000							2,400,000			2,377,000	5,627,000
Purchase price of properties																	200,700,000					200,700,000	200,700,000
Acquired certain interests in producing oil and natural gas assets																	200,700,000					200,700,000	200,700,000
Commencement of Interest payable on Senior notes															Feb. 15, 2013	Aug. 15, 2012	Aug. 15, 2011
Line of Credit Facility, Maximum Borrowing Capacity						750,000,000	250,000,000		250,000,000
Borrowing base under the expanded credit facility						425,000,000		200,000,000
Margin on borrowings under Senior Credit Facility which bear interest ranging from a base rate											0.75%	1.75%
Margin on senior credit facility which bear interest ranging from LIBOR											1.75%	2.75%
Amounts drawn under Senior Credit Facility					195,000,000	205,000,000
Availability under Senior Credit Facility					425,000,000
Senior credit facility condition					Our obligations under our Senior Credit Facility, as well as any hedging contracts and treasury management agreements with the lenders or affiliates of lenders, are secured by substantially all of our assets, including a) mortgages on at least 80% of the total value of our oil and gas properties evaluated in the most recently completed reserve report, after giving effect to exploration and production activities, acquisitions and dispositions, and b) the stock of certain wholly-owned subsidiaries.
Debt redemption notice																				30 days	60 days
Redeemable debt, redemption percentage of principal prior to 2014																					35.00%
Early debt redemption notice																				30 days	60 days
Redeemable debt, redemption percentage prior to 2014																			108.25%
Redeemable debt, percentage of principal outstanding prior to 2014																				65.00%
Redeemable debt, redemption period after equity offering																			90 days
Minimum debt owned for repurchase right																			2,000
Minimum debt owned for repurchase right, additional increments																			1,000
Repurchase right, redemption percentage																			101.00%
Repurchase period after sale of assets																			360 days
Minimum proceeds from sale of assets that must be applied to senior notes																			20,000,000
Repurchase Percentage of Senior Notes																			100.00%
Commitment fees											0.375%	0.50%
Line of Credit Facility Maximum Letter of Credit Capacity					20,000,000
Covenant certification requirements					Our Senior Credit Facility contains customary covenants, default provisions and collateral requirements. As described in the agreement underlying our Senior Credit Facility, we must maintain, for each period for which a covenant certification is required, (a) a minimum current ratio (as defined in the agreement for our Senior Credit Facility) of 1.0 to 1.0 and (b) a maximum total debt to EBITDAX ratio of 3.5 to 1.0. We are also required to maintain a commodities hedging program that is in compliance with the requirements set forth in our Senior Credit Facility. Our Senior Credit Facility also places restrictions on the maximum estimated future production volumes that can be subject to commodity derivative instruments.
Effective interest rate																9.20%		9.00%
Accrued interest included in purchase price		$ 6,500,000		$ 16,200,000											$ 4,800,000