Shareholders' equity - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	$ 2,066,753	$ 1,927,640	$ 1,790,573
Other comprehensive income (loss), net of taxes	(1,372)	(6,867)	1,116
Reclass of AOCI for tax rate reduction impact	0
Ending Balance	2,097,386	2,066,753	1,927,640
Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	1,799,787	1,728,325	1,682,144
Other comprehensive income (loss), net of taxes	(688)	(1,247)	880
Reclass of AOCI for tax rate reduction impact	0
Ending Balance	1,845,283	1,799,787	1,728,325
AOCI
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(33,129)	(26,262)	(27,378)
Other comprehensive income (loss), net of taxes	(1,372)	(6,867)	1,116
Reclass of AOCI for tax rate reduction impact	(7,440)
Ending Balance	(41,941)	(33,129)	(26,262)
AOCI \| Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(322)	925	45
Other comprehensive income (loss), net of taxes	(688)	(1,247)	880
Reclass of AOCI for tax rate reduction impact	(209)
Ending Balance	(1,219)	(322)	925
Net unrealized gains (losses) on securities
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(7,931)	(1,872)	462
Other comprehensive income (loss), net of taxes	(4,370)	(6,059)	(2,334)
Reclass of AOCI for tax rate reduction impact	(2,650)
Ending Balance	(14,951)	(7,931)	(1,872)
Unrealized gains (losses) on derivatives
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(454)	(54)	(289)
Other comprehensive income (loss), net of taxes	454	(400)	235
Reclass of AOCI for tax rate reduction impact	0
Ending Balance	0	(454)	(54)
Unrealized gains (losses) on derivatives \| Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(454)	0	0
Other comprehensive income (loss), net of taxes	454	(454)	0
Reclass of AOCI for tax rate reduction impact	0
Ending Balance	0	(454)	0
Retirement benefit plans
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	(24,744)	(24,336)	(27,551)
Other comprehensive income (loss), net of taxes	2,544	(408)	3,215
Reclass of AOCI for tax rate reduction impact	(4,790)
Ending Balance	(26,990)	(24,744)	(24,336)
Retirement benefit plans \| Hawaiian Electric Company, Inc. and Subsidiaries
AOCI Attributable to Parent [Roll Forward]
Beginning Balance	132	925	45
Other comprehensive income (loss), net of taxes	(1,142)	(793)	880
Reclass of AOCI for tax rate reduction impact	(209)
Ending Balance	$ (1,219)	$ 132	$ 925

AOCI Attributable to Parent [Roll Forward]