Summary of significant accounting policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity method
Composite annual depreciation rate (as a percent)	3.20%	3.20%	3.10%
Leases
Operating lease expense	$ 19	$ 18	$ 19
Future minimum lease payments
2017	12
2018	9
2019	7
2020	5
2021	4
Thereafter	8
Total	45
Hawaiian Electric Company, Inc. and Subsidiaries
Leases
Operating lease expense	10	$ 9	$ 9
Future minimum lease payments
2017	6
2018	4
2019	4
2020	3
2021	3
Thereafter	4
Total	$ 24
Maximum
Cash and cash equivalents
Original maturity period of investments classified as cash equivalents	3 months
Equity method
Ownership percentage of affiliates classified as equity method investments	50.00%