Business Combination, Number of Regulated Water Utilities

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Solar Power Capacity

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Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Price of a single share of a number of saleable stocks paid or offered to be paid in a business combination.

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Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

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Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

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This element represents the amount of earnings or loss of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

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This element represents the amount of revenue of the acquiree since the acquisition date included in the consolidated income statement for the reporting period.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

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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.

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The cash outflow associated with the acquisition of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale; includes cash outflows to pay for construction of self-constructed assets.

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The cash inflow from a contractual arrangement with the lender, including letter of credit, standby letter of credit and revolving credit arrangements, under which borrowings can be made up to a specific amount at any point in time with maturities due beyond one year or the operating cycle, if longer.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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Business acquisitions and development projects - Additional Information (Details) $ / shares in Units, CAD in Thousands, $ in Thousands													3 Months Ended		12 Months Ended
	Sep. 18, 2017 USD ($)	Feb. 28, 2017 USD ($)	Feb. 17, 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 ($)	Jan. 08, 2016 CAD water_utility	Jan. 08, 2016 USD ($) water_utility	Aug. 12, 2015 MWac	Mar. 31, 2018 CAD	Dec. 31, 2016 CAD MWh $ / kWh	Dec. 31, 2018 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2017 USD ($)	Dec. 31, 2016 CAD	Mar. 31, 2018 $ / shares
Business Acquisition [Line Items]
Additions to property, plant and equipment \| CAD																CAD 740,023		CAD 405,743
Property, plant and equipment \| CAD														CAD 4,889,946		7,909,493		4,889,946
Intangible assets \| CAD														64,989		CAD 64,108		64,989
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	39 years
Park Water Company
Business Acquisition [Line Items]
Business combination, purchase price										CAD 353,077	$ 249,540
Business combination, liabilities incurred											$ 91,750
Number of regulated water utilities \| water_utility										3	3
Revenue \| CAD																CAD 91,817		96,695
Pre-tax net earnings \| CAD																CAD 17,620		CAD 25,374
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																	$ 42,750
Great Bay Solar Facility \| Partnership \| Tax Investor
Business Acquisition [Line Items]
Partnership agreement, funded amount	$ 59
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			$ 31,212			$ 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
Wind Turbine Components
Business Acquisition [Line Items]
Additions to property, plant and equipment \| CAD														CAD 75,000
Renewable energy production tax credit (usd per kwh) \| $ / kWh														0.023
Acquisition Facility \| Line of Credit
Business Acquisition [Line Items]
Proceed from long-term lines of credit							$ 1,336,440
Minimum \| Wind Turbine Components
Business Acquisition [Line Items]
Wind power capacity (megawatt) \| MWh														500
Maximum \| Wind Turbine Components
Business Acquisition [Line Items]
Wind power capacity (megawatt) \| MWh														700
Weighted Average \| Park Water Company
Business Acquisition [Line Items]
Estimated useful lives										40 years	40 years
Bridge Loan \| Acquisition Facility \| Line of Credit
Business Acquisition [Line Items]
Proceed from long-term lines of credit					$ 1,336,440		$ 1,336,440
Scenario, Forecast [Member] \| Perris Water Distribution System
Business Acquisition [Line Items]
Business combination, purchase price															$ 12
Scenario, Forecast [Member] \| St. Lawrence Gas Company, Inc.
Business Acquisition [Line Items]
Business combination, purchase price															$ 70
Atlantica Yield \| Scenario, Forecast [Member]
Business Acquisition [Line Items]
Equity interest																			25.00%
Total purchase price \| CAD													CAD 608
Equity Method Investment, Share Price \| $ / shares																			$ 24.25
Business Acquisition, Share Price, Contingent Payment \| $ / shares																			$ 0.60