Credit agreements and changes in long-term debt - Note purchase agreements (Details) - Senior Notes	Sep. 30, 2021 USD ($)
HEI Series 2021A
Debt Instrument [Line Items]
Aggregate principal amount	$ 30,000,000
Fixed coupon interest rate	2.48%
HEI Series 2021B
Debt Instrument [Line Items]
Aggregate principal amount	$ 25,000,000
Fixed coupon interest rate	2.78%
HEI Series 2021C
Debt Instrument [Line Items]
Aggregate principal amount	$ 20,000,000
Fixed coupon interest rate	3.74%
HEI Series 2022A
Debt Instrument [Line Items]
Aggregate principal amount	$ 30,000,000
Fixed coupon interest rate	2.98%
HEI Series 2022B
Debt Instrument [Line Items]
Aggregate principal amount	$ 20,000,000
Fixed coupon interest rate	3.94%