Income taxes - Deferred Tax Assets and Liabilities Due to Book/Tax Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Regulatory liabilities, excluding amounts attributable to property, plant and equipment	$ 104,868	$ 104,984
Allowance for bad debts	14,647	16,192
Other	46,036	24,397
Total deferred tax assets	165,551	145,573
Deferred tax liabilities
Property, plant and equipment related	437,644	415,452
Regulatory assets, excluding amounts attributable to property, plant and equipment	37,345	38,314
Deferred RAM and RBA revenues	11,278	15,038
Retirement benefits	20,173	32,952
Other	31,629	32,247
Total deferred tax liabilities	538,069	534,003
Net deferred income tax liability	372,518	388,430
Hawaiian Electric Company, Inc. and Subsidiaries
Deferred tax assets
Regulatory liabilities, excluding amounts attributable to property, plant and equipment	104,868	104,984
Allowance for bad debts	659	1,812
Other	26,522	11,253
Total deferred tax assets	132,049	118,049
Deferred tax liabilities
Property, plant and equipment related	434,831	413,891
Regulatory assets, excluding amounts attributable to property, plant and equipment	37,345	38,314
Deferred RAM and RBA revenues	11,278	15,038
Retirement benefits	25,430	38,020
Other	6,362	6,827
Total deferred tax liabilities	515,246	512,090
Net deferred income tax liability	$ 383,197	$ 394,041