Electric utility segment - Unconsolidated variable interest entities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) agreement entity	Dec. 31, 2017 USD ($)
Hawaiian Electric Company
Power purchase agreement
Number of power purchase agreements (PPAs) (in agreements) | agreement	5
Number of entities currently required to be consolidated as VIEs (in entities) | entity	0
Number of IPPs | entity	3
Number of firm capacity producers declining to provide financial information to determine primary beneficiary status (in entities) | entity	2
Minimum potential number of IPP entities consolidated into company in the future (in entities) | entity	1
Variable Interest Entity, Not Primary Beneficiary
Unconsolidated variable interest entities
Investment in 2004 Debentures	$ 51,500	$ 51,500
Interest income	$ 800
Variable Interest Entity, Not Primary Beneficiary | Hawaiian Electric Company
Unconsolidated variable interest entities
Percent of ownership in Trust III (as a percent)	100.00%
Variable Interest Entity, Not Primary Beneficiary | Hawaiian Electric Company | 2004 Trust Preferred Securities
Unconsolidated variable interest entities
Balance of Trust Securities	$ 50,000	50,000
Dividend distributions on Trust Preferred Securities	800
Variable Interest Entity, Not Primary Beneficiary | Hawaiian Electric Company | Trust Common Securities
Unconsolidated variable interest entities
Balance of Trust Securities	1,500	$ 1,500
Common dividend	$ 25