Acquisition and Disposition of Properties - Pro Forma Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues	$ 221,858us-gaap_SalesRevenueNet	$ 297,431us-gaap_SalesRevenueNet
Income (loss) from continuing operations	(15,420)	15,647
Net Income (Loss)	(15,420)us-gaap_NetIncomeLoss	53,316us-gaap_NetIncomeLoss
Diluted earnings per average common share from continuing operations	$ (0.21)	$ 0.21
Diluted earnings per average common share	$ (0.21)us-gaap_EarningsPerShareDiluted	$ 0.73us-gaap_EarningsPerShareDiluted
Basic earnings per average common share from continuing operations	$ (0.21)	$ 0.21
Basic earnings per average common share	$ (0.21)us-gaap_EarningsPerShareBasic	$ 0.73us-gaap_EarningsPerShareBasic
San Juan Basin \| Pro Forma
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenues	198,213	243,090
Income (loss) from continuing operations	(33,495)	1,849
Net Income (Loss)	$ (33,495)	$ 39,518
Diluted earnings per average common share from continuing operations	$ (0.46)	$ 0.03
Diluted earnings per average common share	$ (0.46)	$ 0.54
Basic earnings per average common share from continuing operations	$ (0.46)	$ 0.03
Basic earnings per average common share	$ (0.46)	$ 0.54

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

Amount of income (loss) from continuing operations attributable to the parent. Also defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests.

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The amount of net income (loss) from continuing operations per each share of common stock or unit outstanding during the reporting period.

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- Definition

The amount of net income (loss) derived from continuing operations during the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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