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%

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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

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