Income taxes - Changes in Unrecognized Tax Benefits (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	$ 3.8	$ 3.6	$ 0.0
Additions based on tax positions taken during the year	0.9	0.0	0.0
Reductions based on tax positions taken during the year	(0.2)	(0.1)	0.0
Additions for tax positions of prior years	0.0	0.3	3.6
Reductions for tax positions of prior years	(0.5)	0.0	0.0
Settlements	0.0	0.0	0.0
Unrecognized tax benefits, at the end of the period	4.0	3.8	3.6
Hawaiian Electric Company, Inc. and Subsidiaries
Changes in total unrecognized tax benefits
Unrecognized tax benefits, at the beginning of the period	3.8	3.6	0.0
Additions based on tax positions taken during the year	0.4	0.0	0.0
Reductions based on tax positions taken during the year	(0.2)	(0.1)	0.0
Additions for tax positions of prior years	0.0	0.3	3.6
Reductions for tax positions of prior years	(0.5)	0.0	0.0
Settlements	0.0	0.0	0.0
Unrecognized tax benefits, at the end of the period	$ 3.5	$ 3.8	$ 3.6

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount of unrecognized tax benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=65882285&loc=SL6600010-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Unrecognized Tax Benefit
-URI http://asc.fasb.org/extlink&oid=6527854

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