Financial Statement Presentation - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 548,710	$ 386,252	$ 1,473,475	$ 1,317,363
Gain (loss) on derivative instruments	(3,537)	(836)	(1,628)	46,353
TOTAL OPERATING REVENUES	545,173	385,416	1,471,847	1,363,716
Natural gas
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	440,835	323,319	1,217,603	1,152,089
Crude oil and condensate
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	0	56,913	48,722	144,528
Brokered natural gas
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	105,849	3,528	203,375	12,260
Other
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 2,026	$ 2,492	$ 3,775	$ 8,486

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The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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