Subsequent Events (Details) (USD $)	1 Months Ended			6 Months Ended				1 Months Ended
	Aug. 31, 2012	Jul. 31, 2012	Jul. 31, 2011	Jun. 30, 2012	Jun. 30, 2011	Aug. 09, 2012	Jul. 30, 2012	Aug. 31, 2012 Revolving Credit Facility [Member]	Jul. 31, 2012 Revolving Credit Facility [Member]	Aug. 09, 2012 Revolving Credit Facility [Member]	Jul. 30, 2012 Revolving Credit Facility [Member]	Jun. 30, 2012 Revolving Credit Facility [Member]	Apr. 11, 2012 Revolving Credit Facility [Member]	Dec. 31, 2011 Revolving Credit Facility [Member]
Senior notes issuance date				Jul. 30, 2012
Senior notes face amount							$ 300,000,000
Senior notes stated interest rate							9.25%
Senior notes, par percentage		98.62%					98.62%
Proceeds from issuance of senior notes		291,200,000
Proceeds from the issuance of senior notes, net of offering costs		290,700,000
Offering costs of senior notes		500,000
Restricted common units awarded to employees			447,351
Price per restricted common unit awarded to employees		$ 18.19
Percentage multiplied by stated principal amount of debt issuance to reduce borrowing base									0.25%
Credit Agreement, reduction in borrowing capacity									75,000,000
Credit Agreement, current borrowing capacity											655,000,000	730,000,000	630,000,000
Credit Agreement, maximum borrowing capacity												1,500,000,000	750,000,000
Credit Agreement, additional borrowings								20,000,000	20,000,000
Credit Agreement, amount outstanding						325,000,000				325,000,000		596,500,000		500,000,000
Credit Agreement, repayment									291,500,000
Payment for early termination of interest rate derivative contracts		15,000,000
Date of purchase and sales agreement				Aug. 03, 2012
Proceeds from the sale of oil and gas assets	$ 2,700,000			$ 240,000	$ 1,327,000