Long-term debt - Changes in Long-term Debt (Details) - Senior notes - USD ($)	Dec. 31, 2020	Sep. 11, 2020
HEI Series 2020A
Debt Instrument [Line Items]
Aggregate principal amount	$ 50,000,000	$ 50,000,000
Fixed coupon interest rate	0.0298%
HEI Series 2020B | January 28, 2021
Debt Instrument [Line Items]
Aggregate principal amount	$ 46,000,000
Fixed coupon interest rate	0.00%
HEI Series 2020B | April 16, 2021
Debt Instrument [Line Items]
Aggregate principal amount	$ 5,000,000
Fixed coupon interest rate	0.0315%
HEI Series 2020C
Debt Instrument [Line Items]
Aggregate principal amount	$ 24,000,000
Fixed coupon interest rate	0.0282%