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Income taxes (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Income Tax Contingency [Line Items] |  |  |
Effective income tax, percent |  | 19.00% | 35.00%
Bonus depreciation, net income tax benefit (expense) |  | $ 5.3 |
Regulatory liabilities | $ 11.3 |  |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |
Income Tax Contingency [Line Items] |  |  |
Effective income tax, percent |  | 19.00% | 36.00%

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- Definition Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(2)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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