Financial Statement Presentation - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 467,650	$ 484,459
Gain on derivative instruments	5,577	33,384
TOTAL OPERATING REVENUES	473,227	517,843
Natural gas
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	412,108	433,442
Crude oil and condensate
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	48,722	42,990
Brokered natural gas
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	4,950	4,695
Other
Disaggregation of Revenue [Line Items]
Total revenues from contracts with customers	$ 1,870	$ 3,332