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Revenues from Contracts with Customers - Schedule of Revenue Attributable to Identified Revenue Streams is Disaggregated (Parenthetical) (Detail) - USD ($) | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Disaggregation of Revenue [Line Items] |  |
Brokered natural gas, marketing and other | $ 59,979,000 | $ 51,648,000
Natural gas sales |  |
Disaggregation of Revenue [Line Items] |  |
Brokered natural gas, marketing and other | 55,900,000 |
Marketing revenues | 3,800,000 |
NGLs sales |  |
Disaggregation of Revenue [Line Items] |  |
Brokered natural gas, marketing and other | $ 304,000 |

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- Definition Revenue related to the downstream oil and gas activities, including the refining of crude oil, as well as the marketing and distribution of crude oil, refined petroleum products, and natural gas. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62246-109447 + Details
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