Credit agreements - Summary of Debt Instruments (Details) - Unsecured Notes - USD ($) $ in Millions	Jul. 18, 2019	Jun. 30, 2019	May 13, 2019
Series 2019A
Debt Instrument [Line Items]
Aggregate principal amount			$ 50.0
Fixed coupon interest rate			4.21%
Series 2019A | Hawaiian Electric (parent only)
Debt Instrument [Line Items]
Aggregate principal amount			$ 30.0
Series 2019A | HELCO
Debt Instrument [Line Items]
Aggregate principal amount			10.0
Series 2019A | Maui Electric
Debt Instrument [Line Items]
Aggregate principal amount			$ 10.0
2004 Junior subordinated deferrable interest debentures redeemed
Debt Instrument [Line Items]
Aggregate principal amount		$ 51.5
Fixed coupon interest rate		6.50%
2004 Junior subordinated deferrable interest debentures redeemed | Hawaiian Electric (parent only)
Debt Instrument [Line Items]
Aggregate principal amount		$ 31.5
2004 Junior subordinated deferrable interest debentures redeemed | HELCO
Debt Instrument [Line Items]
Aggregate principal amount		10.0
2004 Junior subordinated deferrable interest debentures redeemed | Maui Electric
Debt Instrument [Line Items]
Aggregate principal amount		$ 10.0
Refunding Series 2019 Special Purpose Revenue Bonds | Subsequent Event
Debt Instrument [Line Items]
Aggregate principal amount	$ 150.0
Fixed coupon interest rate	3.20%
Refunding Series 2019 Special Purpose Revenue Bonds | Hawaiian Electric (parent only) | Subsequent Event
Debt Instrument [Line Items]
Aggregate principal amount	$ 90.0
Refunding Series 2019 Special Purpose Revenue Bonds | HELCO | Subsequent Event
Debt Instrument [Line Items]
Aggregate principal amount	60.0
Series 2009 Special Purpose Revenue Bonds Redeemed | Subsequent Event
Debt Instrument [Line Items]
Aggregate principal amount	$ 150.0
Fixed coupon interest rate	6.50%
Series 2009 Special Purpose Revenue Bonds Redeemed | Hawaiian Electric (parent only) | Subsequent Event
Debt Instrument [Line Items]
Aggregate principal amount	$ 90.0
Series 2009 Special Purpose Revenue Bonds Redeemed | HELCO | Subsequent Event
Debt Instrument [Line Items]
Aggregate principal amount	$ 60.0