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Equity Method Investments - Narrative (Details) |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Aug. 14, 2018 USD ($) decathermal / d | Dec. 31, 2018 USD ($) investment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2018 USD ($) | Feb. 28, 2014 | Apr. 30, 2012 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Number of investments | investment |  | 2 |  |  |  |  |
Contributions |  | $ 77,263,000 | $ 57,039,000 | $ 28,484,000 |  |  |
Transcontinental Gas Pipe Line Company LLC |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Entity's equity interest in construction agreement (as a percent) |  |  |  |  |  | 61.00% |
Firm transportation capacity | decathermal / d | 250,000 |  |  |  |  |  |
Constitution Pipeline Company, LLC |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  | 25.00%
Contribution cap |  |  |  |  |  |  | $ 250,000,000
Equity method investment, difference between carrying and underlying investment |  |  |  |  | $ 95,900,000 |  |
Contributions |  | 500,000 | 4,350,000 | 8,975,000 |  |  |
Meade Pipeline Co LLC |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  | 20.00% |
Entity's equity interest in construction agreement (as a percent) |  |  |  |  |  | 39.00% |
Contributions | $ 17,800,000 | $ 76,763,000 | $ 52,689,000 | $ 19,509,000 |  |  |

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- Definition Difference between amount at which an investment accounted for under the equity method of accounting is carried (reported) on the balance sheet and amount of underlying equity in net assets the reporting Entity has in the investee. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571 + Details
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- Definition The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3213-108585 + Details
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