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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