Debt (Details) (USD $)	12 Months Ended		12 Months Ended			12 Months Ended					12 Months Ended					12 Months Ended							12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2012 Senior Credit Agreement item	Dec. 31, 2011 Senior Credit Agreement	Jul. 15, 2005 Senior Credit Agreement	Dec. 31, 2012 Senior Credit Agreement Minimum	Dec. 31, 2012 Senior Credit Agreement Maximum	Dec. 31, 2012 Senior Credit Agreement LIBOR	Dec. 31, 2012 Senior Credit Agreement LIBOR Minimum	Dec. 31, 2012 Senior Credit Agreement LIBOR Maximum	Dec. 31, 2012 Senior Credit Agreement Base Rate	Dec. 31, 2012 Senior Credit Agreement Base Rate Minimum	Dec. 31, 2012 Senior Credit Agreement Base Rate Maximum	Dec. 31, 2012 Second Lien Credit Agreement	Dec. 27, 2010 Second Lien Credit Agreement	Dec. 31, 2012 Second Lien Credit Agreement Minimum	Dec. 31, 2012 Second Lien Credit Agreement LIBOR	Dec. 31, 2012 Second Lien Credit Agreement Base Rate	Dec. 31, 2012 Second Lien Credit Agreement One month LIBOR	Dec. 31, 2012 Second Lien Credit Agreement Federal Funds Effective Rate	Dec. 31, 2012 Second Lien Credit Agreement LIBOR floor	Dec. 31, 2012 Letter-of-credit	Dec. 31, 2012 Senior unsecured notes	Dec. 31, 2012 Senior unsecured notes Senior Credit Agreement
Debt
Maximum borrowing capacity					$ 400,000,000																			$ 200,000,000
Debt instrument covenant, current ratio						1.0
Number of trailing fiscal quarters used in calculation of debt ratios	4
Debt instrument covenant leverage ratio							3.50
Cash Interest Coverage Ratio						2.75
Debt instrument covenant revolving capacity leverage ratio							2.25
Borrowing base			100,000,000
Reduction in current borrowing base as a percentage of net proceeds from issuance of senior unsecured notes in excess of $150 million																								25.00%
Amount borrowed under debt agreement above which there will be deduction in borrowing base																							150,000,000
Number of additional unscheduled redeterminations between scheduled redetermination dates			1
Sub-limit for the issuance of letters-of-credit under the credit agreement																						5,000,000
Amount outstanding			69,200,000																			67,600
Interest rate margin (as a percent)									1.75%	2.75%		0.75%	1.75%				9.50%	8.50%	1.00%	0.50%	3.00%
Variable rate basis								one, two, three and nine month LIBOR			base rate						LIBOR		one month	Federal Funds Effective Rate
Commitment fee (as a percent)						0.375%	0.50%
Remaining borrowing capacity			30,800,000
Principal amount of debt															175,000,000
Specified interest rate (as a percent)																		4.00%
PV-10 value to total net debt ratio																1.50
Principal amount outstanding, gross	244,102,161	195,977,937	69,102,161	20,977,937										175,000,000
Discount on debt instrument	4,733,296	5,936,004												4,700,000
Net outstanding balance			$ 69,102,161	$ 20,977,937										$ 170,300,000