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Business acquisitions and development projects - Additional Information (Details) $ / shares in Units, customer in Thousands, $ in Thousands |  |  |  |  |  |  |  |  |  |  | 1 Months Ended | 12 Months Ended
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Dec. 04, 2018 USD ($) customer km | Aug. 31, 2017 USD ($) | Aug. 10, 2017 USD ($) | Feb. 28, 2017 USD ($) | Feb. 15, 2017 MWac | Jan. 01, 2017 USD ($) $ / shares | Dec. 31, 2016 USD ($) | Dec. 30, 2016 USD ($) | Dec. 14, 2016 MWac | Nov. 29, 2016 USD ($) | Mar. 31, 2018 MWac | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
New Brunswick Gas |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of customers providing natural gas to | customer | 12,000 |  |  |  |  |  |  |  |  |  |  |  |
Length of natural gas distribution pipeline | km | 800 |  |  |  |  |  |  |  |  |  |  |  |
Business combination, purchase price | $ 331,000 |  |  |  |  |  |  |  |  |  |  |  |
Perris Water Distribution System |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Business combination, purchase price |  |  | $ 11,500 |  |  |  |  |  |  |  |  |  |
St. Lawrence Gas Company, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Business combination, purchase price |  | $ 70,000 |  |  |  |  |  |  |  |  |  |  |
Empire |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Business combination, purchase price |  |  |  |  |  | $ 2,414,000 |  |  |  |  |  |  |
Business acquisition, cash paid per common share (usd per share) | $ / shares |  |  |  |  |  | $ 34.00 |  |  |  |  |  |  |
Business combination, liabilities incurred |  |  |  |  |  | $ 855,000 |  |  |  |  |  |  |
Intangible asset, useful life |  |  |  |  |  |  |  |  |  |  |  | 39 years |
Great Bay Solar Project |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Solar power capacity (megawatt ac) | MWac |  |  |  |  |  |  |  |  |  |  | 75 |  |
Great Bay Solar Facility | Partnership |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Partnership agreement, funded amount |  |  |  |  |  |  |  |  |  |  |  | $ 15,250 | $ 42,750
Luning Solar Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Solar power capacity (megawatt ac) | MWac |  |  |  |  | 50 |  |  |  |  |  |  |  |
Luning Solar Facility | Partnership | Tax Investor |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Partnership agreement, funded amount |  |  |  |  |  |  | $ 7,826 |  |  |  |  |  |
Bakersfield Solar Project |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Solar power capacity (megawatt ac) | MWac |  |  |  |  |  |  |  |  | 20 |  |  |  |
Bakersfield Solar Project II |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Solar power capacity (megawatt ac) | MWac |  |  |  |  |  |  |  |  | 10 |  |  |  |
Bakersfield Solar Project II | Tax Investor |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Partnership agreement, funded amount |  |  |  | $ 9,800 |  |  |  |  |  | $ 2,454 |  |  |
Bridge Loan | Acquisition Facility | Line of Credit |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceed from long-term lines of credit |  |  |  |  |  |  |  | $ 1,336,440 |  |  |  |  |
Bridge Loan | Acquisition Facility | Line of Credit | Empire |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceed from long-term lines of credit |  |  |  |  |  | $ 1,336,440 |  |  |  |  |  |  |

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