Summary of significant accounting policies - General, Cash, and PPE (Details)			12 Months Ended
	Feb. 02, 2018 MWh MW	Nov. 24, 2017 MW	Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Accounting Policies [Line Items]
Restricted cash | $			$ 30,872,000	$ 0	$ 0
Composite annual depreciation rate			3.20%	3.20%	3.20%
Minimum
Significant Accounting Policies [Line Items]
Estimated useful life under production plant (in years)			16 years
Estimated useful life under transmission and distribution plant (in years)			10 years
Estimated useful life under general plant (in years)			5 years
Maximum
Significant Accounting Policies [Line Items]
Estimated useful life under production plant (in years)			88 years
Estimated useful life under transmission and distribution plant (in years)			79 years
Estimated useful life under general plant (in years)			65 years
Hamakua Energy Partners, L.P. (HEP)
Significant Accounting Policies [Line Items]
Power produced by power plants (in megawatts) | MW		60	60
Mauo, LLC
Significant Accounting Policies [Line Items]
Power produced by power plants (in megawatts) | MW	8.6
Power storage capacity (in megawatt hours) | MWh	42.3
American Savings Bank (ASB)
Significant Accounting Policies [Line Items]
Reserve requirement | $			$ 26,200,000	$ 28,100,000