Summary of significant accounting policies - Depreciation (Details) $ in Millions			12 Months Ended
	Feb. 02, 2018 MWh MW	Nov. 24, 2017 MW	Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016
Depreciation
Composite annual depreciation rate			3.20%	3.20%	3.20%
Minimum
Depreciation
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
Depreciation
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)
Depreciation
Power produced by power plants (in megawatts)		60	60
Mauo, LLC
Depreciation
Power produced by power plants (in megawatts)	8.6
Power storage capacity (in megawatt hours) | MWh	42.3
American Savings Bank (ASB)
Depreciation
Reserve requirement | $			$ 28.1	$ 17.9