Shareholders' equity - Reclassification out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	$ 524	$ 762
Hawaiian Electric Company, Inc. and Subsidiaries
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	31	459
Amortization of prior service credit and net losses recognized during the period in net periodic benefit cost
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	5,146	3,921
Amortization of prior service credit and net losses recognized during the period in net periodic benefit cost | Hawaiian Electric Company, Inc. and Subsidiaries
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	4,653	3,618
Impact of D&Os of the PUC included in regulatory assets
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	(4,622)	(3,613)
Impact of D&Os of the PUC included in regulatory assets | Hawaiian Electric Company, Inc. and Subsidiaries
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	(4,622)	(3,613)
Reclassification out of Accumulated Other Comprehensive Income | Unrealized gains (losses) on derivatives | Forward commitments | Hawaiian Electric Company, Inc. and Subsidiaries
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	0	454
Reclassification out of Accumulated Other Comprehensive Income | Unrealized gains (losses) on derivatives | Electric utility | Forward commitments
Reclassifications out of accumulated other comprehensive income/(loss)
Total reclassifications	$ 0	$ 454