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Electric utility subsidiary - Commitments and contingencies (Details) |  |  |  |  |  |  |  |  |  | 1 Months Ended | 6 Months Ended |  |  |  |
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Jun. 18, 2015 | May. 28, 2015 USD ($) kWh | Apr. 15, 2015 USD ($) | Mar. 31, 2015 USD ($) | Mar. 23, 2015 | Jan. 20, 2015 | Dec. 19, 2014 memorandum | May. 31, 2013 | Feb. 16, 2012 generation_unit | Jun. 30, 2015 USD ($) MW | Aug. 31, 2014 | Apr. 30, 2014 order | Aug. 31, 2012 MW | Mar. 31, 1988 MW | Jun. 30, 2015 USD ($) | Jun. 30, 2014 USD ($) | Dec. 31, 2014 USD ($) | Mar. 01, 2014 | Feb. 07, 2014 | Mar. 31, 2012 USD ($)
Regulatory projects and legal obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase commitment, period |  |  |  |  |  |  |  |  |  |  |  |  |  | 30 years |  |  |  |  |  |
Long-term Purchase Commitment, Minimum Power Volume Required | MW |  |  |  |  |  |  |  |  |  |  |  |  | 186 | 180 |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of EGUs impacted by proposed rules of MATS | generation_unit |  |  |  |  |  |  |  |  | 14 |  |  |  |  |  |  |  |  |  |  |
Period for which operations was stopped prior to merger |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |
Number of memoranda issued | memorandum |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |
Decrease in GHG emission level regulatory requirements | 16.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of orders from regulatory agency | order |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |
Percent of energy production from renewable energy sources |  |  |  |  |  |  |  |  |  |  | 65.00% |  |  |  |  |  |  |  |  |
Requested approval period for motion |  |  |  |  |  | 60 days |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Phase-in Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual incremental RAM adjusted revenues |  | $ 11,100,000 | $ 26,200,000 | $ 31,600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net annual incremental amount to be collected under the tariffs |  | 400,000 | $ 14,700,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long Term Purchase Commitment, Additional Capacity Requirement | MW |  |  |  |  |  |  |  |  |  | 9 |  |  |  |  |  |  |  |  |  |
Hawaiian Electric Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of extension resulting in MATS compliance date |  |  |  |  |  |  |  |  | 1 year |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Phase-in Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual incremental RAM adjusted revenues |  | 8,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual change in accrued earnings sharing credits to be refunded |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual change in accrued RBA balance as of December 31, 2014 (and associated revenue taxes) to be collected |  | (9,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net annual incremental amount to be collected under the tariffs |  | (1,100,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impact on typical residential customer monthly bill |  | $ (90,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Monthly utility usage assumption (in kilowatts per hour) | kWh |  | 500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lanai and Molokai [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Phase-in Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impact on typical residential customer monthly bill |  | $ 0.11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Monthly utility usage assumption (in kilowatts per hour) | kWh |  | 400 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Hawaii Electric Light Company, Inc. (HELCO) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Phase-in Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual incremental RAM adjusted revenues |  | $ 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual change in accrued earnings sharing credits to be refunded |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual change in accrued RBA balance as of December 31, 2014 (and associated revenue taxes) to be collected |  | 100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net annual incremental amount to be collected under the tariffs |  | 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impact on typical residential customer monthly bill |  | $ 880,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Monthly utility usage assumption (in kilowatts per hour) | kWh |  | 500 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maui Electric |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional accrued investigation and estimated cleanup costs |  |  |  |  |  |  |  |  |  | $ 3,600,000 |  |  |  |  | $ 3,600,000 |  |  |  |  | $ 3,100,000
Public Utilities, Phase-in Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual incremental RAM adjusted revenues |  | $ 1,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual change in accrued earnings sharing credits to be refunded |  | (100,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual change in accrued RBA balance as of December 31, 2014 (and associated revenue taxes) to be collected |  | (2,200,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net annual incremental amount to be collected under the tariffs |  | (800,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impact on typical residential customer monthly bill |  | $ (130,000) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of reduction in GHG emissions by 2020 |  |  |  |  |  |  |  |  |  | 16.00% |  |  |  |  | 16.00% |  |  |  |  |
Estimated annual fee for greenhouse gas emissions |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |
Impact on earnings from recognition of AROs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Balance, beginning of period |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 29,419,000 | $ 43,106,000 |  |  |  |
Accretion expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,000 | 643,000 |  |  |  |
Liabilities incurred |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |
Liabilities settled |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (1,881,000) | (6,052,000) |  |  |  |
Revisions in estimated cash flows |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |
Balance, end of period |  |  |  |  |  |  |  |  |  | $ 27,550,000 |  |  |  |  | $ 27,550,000 | $ 37,697,000 |  |  |  |
Decoupling implementation experience, period |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |
Proposed rate base adjustment, percent of previous rate base adjustment |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 90.00% |
Effective interest rate, revenue balancing account |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |
Period to file required plan |  |  |  |  |  |  |  |  |  |  |  | 120 days |  |  |  |  |  |  |  |
Period to file implementation plan |  |  |  |  |  |  |  |  |  |  |  | 60 days |  |  |  |  |  |  |  |
Period for full implementation of plan |  |  |  |  |  |  |  |  |  |  |  | 180 days |  |  |  |  |  |  |  |
Period to file action plan |  |  |  |  |  |  |  |  |  |  |  | 30 days |  |  |  |  |  |  |  |
Proposal to file implementation proposal |  |  |  |  |  |  |  |  |  |  |  | 120 days |  |  |  |  |  |  |  |
Period to file demand response portfolio plan |  |  |  |  |  |  |  |  |  |  |  | 90 days |  |  |  |  |  |  |  |
Minimum | Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed effective interest rate, revenue balancing account |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.25% |  |
Maximum | Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed effective interest rate, revenue balancing account |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.25% |  |
Reserve for Environmental Costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional reserve for investigation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 800,000 |  |  |
PCB Contamination | Waiau Power Plant |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period for submissions to EPA |  |  |  |  | 45 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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