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Retirement benefits - Additional Information (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |
Vested percentage of interest of each affected participant after participating employer terminates participation | 100.00% |  |
Number of years for which regulatory asset/liability for each utility will be amortized, beginning with respective utility's next rate case | 5 years |  |
Executive life and nonqualified pension plan expenses | $ 1,100,000 | $ 900,000 |
Regulatory asset charges pretax | $ (128,000,000) | 47,000,000 |
Fair value of plan assets, valuation difference amortized in first year | 0.00% |  |
Fair value of plan assets, valuation difference amortized in two to five years | 25.00% |  |
Number of past years for adding or subtracting the unamortized differences from fair value | 4 years |  |
Percentage of range around fair value | 15.00% |  |
Expected cash funding for qualified defined benefit plans |  |  |
Defined contribution plan, expenses recognized | $ 7,000,000 | 5,000,000 | $ 6,000,000
Cash contributions by the employer to defined contribution plan | $ 6,000,000 | 5,000,000 | 5,000,000
Pension benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Expected rate of return on plan assets | 7.50% |  |
Discount rate | 3.94% |  |
Assumed discount rate, active manager premium | 0.20% |  |
Pension expense | $ 33,000,000 | 33,000,000 | 35,000,000
Defined benefit pension plans with accumulated benefit obligations in excess of plan assets |  |  |
Defined benefit plans - ABOs | 1,800,000,000 | 1,700,000,000 |
Aggregate accumulated benefit obligations | 1,700,000,000 | 1,600,000,000 |
Plan assets | 1,500,000,000 | 1,300,000,000 |
Defined benefit plans with benefit obligations in excess of plan assets |  |  |
Aggregate projected benefit obligations | 2,000,000,000 | 1,800,000,000 |
Plan assets | 1,500,000,000 | $ 1,300,000,000 |
Expected cash funding for qualified defined benefit plans |  |  |
Estimate of contributions to postretirement benefit plans in 2018 | $ 62,000,000 |  |
Other benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Assumed health care trend rate for medical in next fiscal year | 7.50% | 7.75% |
Assumed health care trend rate for grading down plan in 12 fiscal years thereafter | 5.00% |  |
Assumed health care trend rate for dental in next fiscal year | 5.00% | 5.00% |
Assumed health care trend rate for vision in next fiscal year | 4.00% | 4.00% |
Assumed health care trend rate for grading down in 2028 and thereafter |  | 5.00% |
Postretirement benefits other than pension expense | $ 1,000,000 | $ 1,000,000 | 900,000
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased total service and interest cost | 100,000 |  |
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased the accumulated postretirement benefit obligation (APBO) | 2,700,000 |  |
Effect of one-percentage-point decrease that would have reduced total service and interest cost | 200,000 |  |
Effect of one-percentage-point decrease that would have reduced APBO | 3,100,000 |  |
Expected cash funding for qualified defined benefit plans |  |  |
Estimate of contributions to postretirement benefit plans in 2018 | 0 |  |
2018 | 86,000,000 |  |
2019 | 89,000,000 |  |
2020 | 92,000,000 |  |
2021 | 95,000,000 |  |
2022 | 99,000,000 |  |
2023 through 2026 | $ 552,000,000 |  |
Minimum |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Threshold percentage around fair value | 85.00% |  |
Maximum |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Threshold percentage around fair value | 115.00% |  |
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |
Expected cash funding for qualified defined benefit plans |  |  |
Maximum percentage of participant deferrals eligible for Employer contribution match, towards defined contribution plan | 6.00% |  |
Employer's additional matching contribution on employee deferrals | 50.00% |  |
Defined contribution plan, expenses recognized | $ 2,000,000 | 1,500,000 | 1,500,000
Hawaiian Electric Company, Inc. and Subsidiaries | Pension benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Actual net return on plan assets | 19.30% |  |
Pension expense | $ 30,000,000 | 30,000,000 | 29,000,000
Defined benefit pension plans with accumulated benefit obligations in excess of plan assets |  |  |
Defined benefit plans - ABOs | 1,700,000,000 | 1,500,000,000 |
Aggregate accumulated benefit obligations | 1,700,000,000 | 1,500,000,000 |
Plan assets | 1,500,000,000 | 1,200,000,000 |
Defined benefit plans with benefit obligations in excess of plan assets |  |  |
Aggregate projected benefit obligations | 1,900,000,000 | 1,800,000,000 |
Plan assets | 1,500,000,000 | 1,200,000,000 |
Expected cash funding for qualified defined benefit plans |  |  |
Estimate of contributions to postretirement benefit plans in 2018 | 61,000,000 |  |
Hawaiian Electric Company, Inc. and Subsidiaries | Other benefits |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Postretirement benefits other than pension expense | 800,000 | $ 700,000 | $ 700,000
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased total service and interest cost | 100,000 |  |
Effect of one-percentage-point increase in assumed health care cost trend rates that would have increased the accumulated postretirement benefit obligation (APBO) | 2,700,000 |  |
Effect of one-percentage-point decrease that would have reduced total service and interest cost | 200,000 |  |
Effect of one-percentage-point decrease that would have reduced APBO | 3,100,000 |  |
Expected cash funding for qualified defined benefit plans |  |  |
Estimate of contributions to postretirement benefit plans in 2018 | 0 |  |
2018 | 79,000,000 |  |
2019 | 81,000,000 |  |
2020 | 84,000,000 |  |
2021 | 87,000,000 |  |
2022 | 90,000,000 |  |
2023 through 2026 | $ 504,000,000 |  |
Hawaii Electric Light Company, Inc. (HELCO) |  |  |
Expected cash funding for qualified defined benefit plans |  |  |
Maximum percentage of participant deferrals eligible for Employer contribution match, towards defined contribution plan | 6.00% |  |
Employer's additional matching contribution on employee deferrals | 50.00% |  |

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- Definition An assumption for the next fiscal year as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References No definition available. + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the dental plan under the defined benefit plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References No definition available. + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the medical plan under the defined benefit plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References No definition available. + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the vision plan under the defined benefit plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References No definition available. + Details
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- Definition Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Grading Down Plan in Nine Fiscal Years Thereafter + References No definition available. + Details
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- Definition Represents the number of years for which regulatory asset/liability for each utilities will be amortized, beginning with the respective utility's next rate case. + References No definition available. + Details
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- Definition The amount of pension benefit costs recognized during the period for defined benefit plans. Pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments, less the impact of Public Utilities Commission decisions and orders and capitalized amounts. + References No definition available. + Details
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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of effect of a one-percentage-point decrease in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the accumulated postretirement benefit obligation for health care benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The effect of a one-percentage-point increase in the assumed health care cost trend rates on the aggregate of the service and interest cost components of net periodic postretirement health care benefit costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition If aggregate disclosures are presented, the aggregate benefit obligation for plans with benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2417-114920 + Details
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- Definition If aggregate disclosures are presented, the aggregate fair value of plan assets for plans with benefit obligations in excess of plan assets as of the measurement date of each statement of financial position presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2417-114920 + Details
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