Summary of significant accounting policies.

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Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
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-Subparagraph (a)
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The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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-Publisher FASB
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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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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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Amount of interest capitalized during the period.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 120,802,000		$ 119,332,000
Capitalized interest expense	$ 700,000	$ 500,000
Number of operating segment \| Segment	1
Oil and Gas Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Depreciation, depletion and amortization	$ 29,500,000	30,900,000
Other property and equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Depreciation	$ 400,000	500,000
Other property and equipment [Member] \| Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, expected lives	5 years
Other property and equipment [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, expected lives	40 years
Proved Oil And Gas Properties [Member] \| Marcellus Shale [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment of oil and gas properties	$ 0	0
Unproved Oil And Gas Properties [Member]
Summary Of Significant Accounting Policies [Line Items]
Impairment of oil and gas properties	9,600,000	$ 6,700,000
Commodity Contract [Member] \| Not Designated as Hedging Instrument [Member]
Summary Of Significant Accounting Policies [Line Items]
Fair value of commodity derivative contracts	(3,300,000)		5,700,000
Revenue From Contract With Customer [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	83,200,000		94,100,000
Unbilled Revenues [Member]
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable	$ 83,200,000		$ 94,100,000