Financial Statement Presentation - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 457,115	$ 446,652	$ 924,765	$ 931,112
Gain (loss) on derivative instruments	(3,668)	13,805	1,909	47,190
TOTAL OPERATING REVENUES	453,447	460,457	926,674	978,302
Natural gas
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	364,660	395,328	776,768	828,770
Crude oil and condensate
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	0	44,625	48,722	87,616
Brokered natural gas
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	92,576	4,037	97,526	8,732
Other
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ (121)	$ 2,662	$ 1,749	$ 5,994

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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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