Indebtedness (Narrative) (Details) (USD $)		6 Months Ended					0 Months Ended		0 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Senior Credit Facility [Member]	Dec. 31, 2012 Senior Credit Facility [Member]	Oct. 31, 2012 Senior Credit Facility [Member]	May 01, 2012 Senior Credit Facility [Member]	Feb. 14, 2011 Senior Credit Facility [Member]	Oct. 25, 2012 2012 Notes [Member]	Jun. 30, 2013 2012 Notes [Member]	Feb. 14, 2011 2011 Notes [Member]	Jun. 30, 2013 8.25% Notes [Member]	Feb. 14, 2011 ASOP Acquisition [Member]
Debt Instrument [Line Items]
Senior Notes, face amount							$ 300,000,000	$ 300,000,000	$ 210,000,000	$ 510,000,000
Interest Rate, Debt Instrument	8.25%									8.25%
Maturity date		Oct. 31, 2016								Feb. 15, 2018
Net proceeds from offering of Notes							289,500,000		202,000,000
Purchase price of properties											200,700,000
Acquired certain interests in producing oil and natural gas assets											200,700,000
Line of Credit Facility, Maximum Borrowing Capacity				750,000,000		250,000,000
Borrowing base under the expanded credit facility		425,000,000		425,000,000	200,000,000
Amounts drawn under Senior Credit Facility		165,000,000	195,000,000
Line of Credit Facility Maximum Letter of Credit Capacity		20,000,000
Effective interest rate										9.10%
Accrued interest included in purchase price							$ 4,800,000