The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Glossary Asset Retirement Obligation
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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount, after accumulated amortization, of debt discount.

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-SubTopic 30
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Amount, after accumulated amortization, of debt issuance costs classified as noncurrent. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Amount of credit risk valuation adjustment to derivative assets to properly reflect the credit quality of the counterparties.

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No definition available.

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The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
-Paragraph 2
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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 1
-Subparagraph (SX 210.5-02(22))
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-Section S99
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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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No definition available.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Sep. 15, 2016
Debt Instrument [Line Items]
Derivative credit risk valuation adjustment, derivative assets	$ 1,200,000		$ 1,200,000		$ 300,000
Long-term debt	2,125,573,000		2,125,573,000		2,302,584,000
Deferred financing costs	35,000,000		35,000,000
Asset retirement obligation, carrying value	37,829,000		37,829,000		35,812,000
Impairment	382,000	$ 80,000	3,967,000	$ 24,917,000
Proved Oil and Gas Properties [Member]
Debt Instrument [Line Items]
Impairment	0	0	0	0
Proved Oil and Gas Properties [Member] \| Assets disposed of by sale [Member]
Debt Instrument [Line Items]
Impairment			3,600,000
Proved Oil and Gas Properties [Member] \| Assets disposed of by sale [Member] \| Operating Segments [Member] \| Midstream Services [Member]
Debt Instrument [Line Items]
Impairment			2,400,000
Proved Oil and Gas Properties [Member] \| Assets disposed of by sale [Member] \| Operating Segments [Member] \| Exploration and Production [Member]
Debt Instrument [Line Items]
Impairment			1,200,000
Proved Oil and Gas Properties [Member] \| Assets held for sale [Member]
Debt Instrument [Line Items]
Impairment	0	0	3,600,000	0	9,400,000
Unproved Oil And Gas Properties [Member] \| Expiring Lease 1 [Member]
Debt Instrument [Line Items]
Impairment	400,000	$ 100,000	400,000	24,900,000
Unproved Oil And Gas Properties [Member] \| Expiring Lease 2 [Member]
Debt Instrument [Line Items]
Impairment	200,000		200,000	$ 16,400,000
Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term debt, fair value	2,018,000,000		2,018,000,000
Line of Credit [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	195,000,000		195,000,000
2.625% senior convertible notes due September 15, 2023 [Member] \| Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt instrument unamortized discount	92,877,000		92,877,000		$ 0	$ 93,200,000
Estimated fair value of debt						$ 206,800,000
Long-term Debt [Member] \| Notes [Member] \| Senior Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term debt	2,053,000,000		2,053,000,000
Debt instrument unamortized discount	92,900,000		92,900,000
Deferred financing costs	$ 29,500,000		$ 29,500,000