Annual report pursuant to Section 13 and 15(d)

Earnings Per Share - Schedule of Calculation of Basic and Diluted Earnings Per Common Share (Details)

Earnings Per Share - Schedule of Calculation of Basic and Diluted Earnings Per Common Share (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Earnings Per Share, Basic and Diluted [Abstract]                      
Net income (loss) $ (33,371)us-gaap_NetIncomeLoss $ 684us-gaap_NetIncomeLoss $ 9,837us-gaap_NetIncomeLoss $ 11,189us-gaap_NetIncomeLoss $ (11,886)us-gaap_NetIncomeLoss [1],[2] $ 14,194us-gaap_NetIncomeLoss [1],[2] $ 22,396us-gaap_NetIncomeLoss [1],[2] $ 26,618us-gaap_NetIncomeLoss [1],[2] $ (11,661)us-gaap_NetIncomeLoss $ 51,322us-gaap_NetIncomeLoss $ 71,984us-gaap_NetIncomeLoss
Less portion allocated to nonvested shares                 269us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic 303us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic 983us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic
Net income (loss) allocated to common shares                 $ (11,930)us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic $ 51,019us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic $ 71,001us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic
Weighted average common shares outstanding                 75,609us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted 75,239us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted 74,354us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted
Basic and diluted earnings (loss) per common share $ (0.44)us-gaap_EarningsPerShareBasicAndDiluted [1] $ 0.01us-gaap_EarningsPerShareBasicAndDiluted [1] $ 0.13us-gaap_EarningsPerShareBasicAndDiluted [1] $ 0.15us-gaap_EarningsPerShareBasicAndDiluted [1] $ (0.16)us-gaap_EarningsPerShareBasicAndDiluted [1],[2] $ 0.19us-gaap_EarningsPerShareBasicAndDiluted [1],[2] $ 0.29us-gaap_EarningsPerShareBasicAndDiluted [1],[2] $ 0.35us-gaap_EarningsPerShareBasicAndDiluted [1],[2] $ (0.16)us-gaap_EarningsPerShareBasicAndDiluted $ 0.68us-gaap_EarningsPerShareBasicAndDiluted $ 0.95us-gaap_EarningsPerShareBasicAndDiluted
Shares excluded due to being anti-dilutive (weighted-average)                 29us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount   1,923us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
[1] The sum of the individual quarterly earnings per share may not agree with year-to-date earnings per share because each quarterly calculation is based on the income for that quarter and the weighted average number of shares outstanding during that quarter.
[2] In January 2014, we identified that we had been receiving an erroneous million British thermal unit (“MMBtu”) conversion factor from a third party that had the effect of understating natural gas production at our Viosca Knoll 783 field (Tahoe).  The incorrect conversion factor had been used on all natural gas production from the field since we acquired it in 2011.  The use of the incorrect conversion factor did not affect revenues, operating cash flows or royalty payments to the federal government but did impact reported natural gas production and the calculation of depletion expense.  We performed an analysis of the information, assessing both quantitative and qualitative factors, and determined that the impact on our net income reported for prior annual periods, as well as the impact to our earnings trend, was not material to 2011 and 2012 results, thus the adjustment was recognized in the fourth quarter of 2013.