Shareholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	$ 2,097,386	$ 2,066,753	$ 1,927,640
Other comprehensive income (loss), net of taxes	(8,669)	(1,372)	(6,867)
Reclass of AOCI for tax rate reduction impact		0
Ending Balance	2,162,280	2,097,386	2,066,753
Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	1,845,283	1,799,787	1,728,325
Other comprehensive income (loss), net of taxes	1,318	(688)	(1,247)
Ending Balance	1,957,641	1,845,283	1,799,787
AOCI
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(41,941)	(33,129)	(26,262)
Other comprehensive income (loss), net of taxes	(8,669)	(1,372)	(6,867)
Reclass of AOCI for tax rate reduction impact		(7,440)
Ending Balance	(50,610)	(41,941)	(33,129)
AOCI | Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(1,219)	(322)	925
Other comprehensive income (loss), net of taxes	1,318	(688)	(1,247)
Reclass of AOCI for tax rate reduction impact		(209)
Ending Balance	99	(1,219)	(322)
Net unrealized gains (losses) on securities
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(14,951)	(7,931)	(1,872)
Other comprehensive income (loss), net of taxes	(9,472)	(4,370)	(6,059)
Reclass of AOCI for tax rate reduction impact		(2,650)
Ending Balance	(24,423)	(14,951)	(7,931)
Unrealized gains (losses) on derivatives
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	0	(454)	(54)
Other comprehensive income (loss), net of taxes	(436)	454	(400)
Reclass of AOCI for tax rate reduction impact		0
Ending Balance	(436)	0	(454)
Unrealized gains (losses) on derivatives | Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	0	(454)	0
Other comprehensive income (loss), net of taxes	0	454	(454)
Reclass of AOCI for tax rate reduction impact		0
Ending Balance	0	0	(454)
Retirement benefit plans
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(26,990)	(24,744)	(24,336)
Other comprehensive income (loss), net of taxes	1,239	2,544	(408)
Reclass of AOCI for tax rate reduction impact		(4,790)
Ending Balance	(25,751)	(26,990)	(24,744)
Retirement benefit plans | Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(1,219)	132	925
Other comprehensive income (loss), net of taxes	1,318	(1,142)	(793)
Reclass of AOCI for tax rate reduction impact		(209)
Ending Balance	$ 99	$ (1,219)	$ 132