Shareholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	$ 2,280,260	$ 2,162,280	$ 2,097,386
Current period other comprehensive income (loss) and reclassifications, net of taxes	18,775	30,571	(8,669)
Ending Balance	2,337,502	2,280,260	2,162,280
Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	2,047,352	1,957,641	1,845,283
Current period other comprehensive income (loss) and reclassifications, net of taxes	(1,640)	(1,378)	1,318
Ending Balance	2,141,918	2,047,352	1,957,641
AOCI
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(20,039)	(50,610)	(41,941)
Current period other comprehensive income (loss) and reclassifications, net of taxes	18,775	30,571	(8,669)
Ending Balance	(1,264)	(20,039)	(50,610)
AOCI | Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(1,279)	99	(1,219)
Current period other comprehensive income (loss) and reclassifications, net of taxes	(1,640)	(1,378)	1,318
Ending Balance	(2,919)	(1,279)	99
Net unrealized gains (losses) on securities
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	2,481	(24,423)	(14,951)
Current period other comprehensive income (loss) and reclassifications, net of taxes	17,505	26,904	(9,472)
Ending Balance	19,986	2,481	(24,423)
Unrealized gains (losses) on derivatives
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(1,613)	(436)	0
Current period other comprehensive income (loss) and reclassifications, net of taxes	(1,750)	(1,177)	(436)
Ending Balance	(3,363)	(1,613)	(436)
Retirement benefit plans
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(20,907)	(25,751)	(26,990)
Current period other comprehensive income (loss) and reclassifications, net of taxes	3,020	4,844	1,239
Ending Balance	(17,887)	(20,907)	(25,751)
Retirement benefit plans | Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(1,279)	99	(1,219)
Current period other comprehensive income (loss) and reclassifications, net of taxes	(1,640)	(1,378)	1,318
Ending Balance	$ (2,919)	$ (1,279)	$ 99