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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)

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- Definition Gain (loss) incurred in hedging activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959 + Details
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- Definition The difference between the book value and the sale price of agreements to purchase or sell mineral resources, energy, and agricultural products at some future point. This element refers to the gain (loss) included in earnings and not to the cash proceeds. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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