Annual report pursuant to Section 13 and 15(d)

Selected Quarterly Financial Data (Details)

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Selected Quarterly Financial Data (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
Quarterly Financial Information Disclosure [Abstract]                      
Revenues $ 196,677us-gaap_Revenues $ 234,521us-gaap_Revenues $ 262,994us-gaap_Revenues $ 254,516us-gaap_Revenues $ 244,928us-gaap_Revenues [1],[2] $ 244,555us-gaap_Revenues [1],[2] $ 235,383us-gaap_Revenues [1],[2] $ 259,222us-gaap_Revenues [1],[2] $ 948,708us-gaap_Revenues $ 984,088us-gaap_Revenues $ 874,491us-gaap_Revenues
Operating income (loss) (30,543)us-gaap_OperatingIncomeLoss 20,983us-gaap_OperatingIncomeLoss 34,403us-gaap_OperatingIncomeLoss 37,225us-gaap_OperatingIncomeLoss 622us-gaap_OperatingIncomeLoss [1],[2] 31,965us-gaap_OperatingIncomeLoss [1],[2] 53,823us-gaap_OperatingIncomeLoss [1],[2] 60,321us-gaap_OperatingIncomeLoss [1],[2] 62,068us-gaap_OperatingIncomeLoss 146,731us-gaap_OperatingIncomeLoss 169,310us-gaap_OperatingIncomeLoss
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
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
[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.