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Summary of Significant Accounting Policies - Investments in Affiliates and Other Entities (Details) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2014 substation kV km
Summary Of Significant Accounting Policies [Line Items] |  |  |
Contributions to partnership in connection with certain investments and management fees | $ 15,100,000 |  |
Water and Gas Pipeline Infrastructure Contractor |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Equity interest acquired |  | 30.00% |
Cash subscription price |  | $ 22,200,000 |
Cash dividends |  | $ 3,900,000 |
Equity Method Investment in Electric Power Infrastructure Services Company |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Equity interest |  | 49.00% |
Payments to acquire equity method investments and intangible assets |  | $ 12,300,000 |
EPC Electric Transmission Project |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Capital commitment | 0 |  |
Length of electrical transmission line to be constructed under contract | km |  |  | 500
Number of substations | substation |  |  | 2
Voltage of substations | kV |  |  | 500
Deferred earnings recognized | 60,300,000 |  |
Capital for Infrastructure Projects |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Capital commitment | 80,000,000.0 |  |
Infrastructure Investors Partnership | Capital for Infrastructure Projects |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Capital commitment | $ 1,000,000,000.0 |  |

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