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The Effect of Derivative Instruments on the Consolidated Statement of Income (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Derivative [Line Items]        
Cash Received (Paid) in Settlement of Commodity Derivative Instruments: $ 16,684 $ (32,285) $ (307) $ (79,388)
Unrealized Gain (Loss) on Commodity Derivative Instruments: 8,976 116,073 61,054 140,713
Gain (Loss) on Commodity Derivative Instruments: 25,660 83,788 60,747 61,325
Basis Swap [Member]        
Derivative [Line Items]        
Cash Received (Paid) in Settlement of Commodity Derivative Instruments: (374) (16,776) (16,931) (38,056)
Unrealized Gain (Loss) on Commodity Derivative Instruments: (37,474) 45,791 11,804 (93,320)
Gain (Loss) on Commodity Derivative Instruments: (37,848) 29,015 (5,127) (131,376)
Natural Gas [Member] | Commodity Swaps [Member]        
Derivative [Line Items]        
Cash Received (Paid) in Settlement of Commodity Derivative Instruments: 17,058 (15,509) 16,624 (40,116)
Unrealized Gain (Loss) on Commodity Derivative Instruments: 46,450 70,282 49,250 232,886
Gain (Loss) on Commodity Derivative Instruments: 63,508 54,773 65,874 192,770
Propane [Member] | Commodity Swaps [Member]        
Derivative [Line Items]        
Cash Received (Paid) in Settlement of Commodity Derivative Instruments: 0 0 0 (1,216)
Unrealized Gain (Loss) on Commodity Derivative Instruments: 0 0 0 1,147
Gain (Loss) on Commodity Derivative Instruments: $ 0 $ 0 $ 0 $ (69)