Credit agreements (Details)	Mar. 31, 2018 USD ($) Institution	Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
Number of financial institutions | Institution	8
Line of credit facility
Credit agreement
Revolving noncollateralized credit facility with a letter of credit sub-facility	$ 150,000,000
Amount outstanding under facilities	0	$ 0
Line of credit facility | Hawaiian Electric Company
Credit agreement
Revolving noncollateralized credit facility with a letter of credit sub-facility	200,000,000
Amount outstanding under facilities	$ 0	$ 0