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Electric utility segment - Commitments and contingencies (Details) |  |  |  |  |  |  |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |
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Nov. 13, 2015 MW | Oct. 30, 2015 USD ($) | Oct. 26, 2015 USD ($) | Aug. 03, 2015 state | May. 28, 2015 USD ($) kWh | Apr. 15, 2015 USD ($) | Mar. 31, 2015 USD ($) | May. 31, 2013 | Feb. 16, 2012 generation_unit | Sep. 30, 2015 USD ($) | Jul. 31, 2015 USD ($) | Jun. 30, 2015 MW | Aug. 31, 2014 | Jul. 31, 2014 USD ($) | Apr. 30, 2014 order | Aug. 31, 2012 MW | Mar. 31, 1988 MW | Sep. 30, 2015 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($) | Dec. 31, 2014 USD ($) | Jun. 20, 2014 facility | Mar. 01, 2014 | Feb. 07, 2014
Regulatory projects and legal obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase commitment, period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30 years |  |  |  |  |  |  |
Minimum power volume required | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 186 | 180 |  |  |  |  |  |  |
Additional capacity requirement | MW |  |  |  |  |  |  |  |  |  |  |  | 9 |  |  |  |  |  |  |  |  |  |  |  |
Enterprise resource management system project, estimated costs |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 82,400,000 |  |  |  |  |  |  |  |  |  |
Enterprise resource management system project, difference between Option A and B |  |  |  |  |  |  |  |  |  |  | $ 20,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |
SAP software costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,800,000 |  |  |  |  |  |
Schofield generating station facility capacity | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50 |  |  |  |  |  |  |  |
Schofield generating station project, budgetary cap |  |  |  |  |  |  |  |  |  | $ 167,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of costs recoverable through recovery mechanisms other than base rates |  |  |  |  |  |  |  |  |  | 90.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |
States included in interim state-wide emission limits | state |  |  |  | 48 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of orders from regulatory agency | order |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |  |
Percent of energy production from renewable energy sources |  |  |  |  |  |  |  |  |  |  |  |  | 65.00% |  |  |  |  |  |  |  |  |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory projects and legal obligations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional capacity requirement | MW | 9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue requirement associated with 2015 Plant additions |  |  | $ 40,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reserve for Environmental Costs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation allowances and reserves |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 800,000 |  |  |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Environmental regulation |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of reduction in GHG emissions by 2020 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 16.00% |  |
Number of facilities affected by GHG rule | facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 11 |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 18,000 | 816,000 |  |  |  |
Liabilities incurred |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |
Liabilities settled |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (2,349,000) | (11,338,000) |  |  |  |
Revisions in estimated cash flows |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | 0 |  |  |  |
Balance, end of period |  |  |  |  |  |  |  |  |  | $ 27,088,000 |  |  |  |  |  |  |  | 27,088,000 | 27,088,000 | $ 32,584,000 |  |  |  |
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%
Public Utilities, Phase-in Plans [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period to file required plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 120 days |  |  |  |  |  |  |  |  |
Hawaiian Electric Company, Inc. and Subsidiaries | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed effective interest rate, revenue balancing account |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.25% |
Hawaiian Electric Company, Inc. and Subsidiaries | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed effective interest rate, revenue balancing account |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.25% |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 | $ 3,600,000 |  |  |  |  |
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) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maui Electric | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in the asset retirement obligation liability |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue requirement associated with 2015 Plant additions |  | $ 4,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lanai and Molokai |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Asset Retirement Obligation -URI http://asc.fasb.org/extlink&oid=6505190 + Details
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- Definition Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848 + Details
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- Definition Amount of asset retirement obligations incurred during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Amount of the Increase or Decrease in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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- Definition Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 09 -Article 12 + Details
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