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Commitments and Contingencies - Investments in Affiliates and Other Entities (Detail) $ in Millions | 3 Months Ended | 33 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 USD ($) | Jun. 30, 2017 USD ($) | May 31, 2022 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2014 kV substation km
Other Commitments, Planned Oil and Gas Infrastructure Projects |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Outstanding capital commitment |  | $ 18.1 |  |  |
Capital for infrastructure projects |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Outstanding capital commitment |  | 60.0 |  |  |
Scenario, Forecast | Other Commitments, Planned Oil and Gas Infrastructure Projects |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Outstanding capital commitment |  |  | $ 17.7 | $ 0.4 |
EPC Electric Transmission Project |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Length of electrical transmission line to be constructed under contract | km |  |  |  |  | 500
Number of substations | substation |  |  |  |  | 2
Voltage of substations | kV |  |  |  |  | 500
Aggregate contributions to this unconsolidated affiliate |  | 26.7 |  |  |
Proceeds from return of capital |  | 3.0 |  |  |
Corporate Joint Venture | Scenario, Forecast | Other Commitments, Planned Oil and Gas Infrastructure Projects |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Proceeds from return of capital | $ 56.3 |  |  |  |
Infrastructure investors partnership | Capital for infrastructure projects |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |
Outstanding capital commitment |  | $ 750.0 |  |  |

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- Definition Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions. + References No definition available. + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3213-108585 + Details
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