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Credit agreements and long-term debt (Details) | 1 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2018 USD ($) | Oct. 04, 2018 USD ($) | Sep. 30, 2018 USD ($) Institution | May 30, 2018 USD ($) | Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract] |  |  |  |  |
Number of financial institutions | Institution |  |  | 8 |  |
Series 2018A | Subsequent Event |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  | $ 50,000,000 |  |  |
Fixed coupon interest rate |  | 4.58% |  |  |
Debt, face amount |  | $ 150,000,000 |  |  |
Series 2018B | Subsequent Event |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  | $ 100,000,000 |  |  |
Fixed coupon interest rate |  | 4.72% |  |  |
Short-term Borrowings with Bank of Tokyo-Mitsubishi | Subsequent Event |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  | $ 50,000,000 |  |  |
Line of credit facility |  |  |  |  |
Credit agreement |  |  |  |  |
Revolving noncollateralized credit facility with a letter of credit sub-facility |  |  | $ 150,000,000 |  |
Amount outstanding under facilities |  |  | 0 |  | $ 0
Line of credit facility | Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |  |
Credit agreement |  |  |  |  |
Revolving noncollateralized credit facility with a letter of credit sub-facility |  |  | 200,000,000 |  |
Amount outstanding under facilities |  |  | 0 |  | $ 0
Unsecured Debt | Series 2018A |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | $ 67,500,000 |
Fixed coupon interest rate |  |  |  | 4.38% |
Unsecured Debt | Series 2018B |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | $ 17,500,000 |
Fixed coupon interest rate |  |  |  | 4.53% |
Unsecured Debt | Series 2018C |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | $ 15,000,000 |
Fixed coupon interest rate |  |  |  | 4.72% |
Unsecured Debt | Hawaiian Electric Company, Inc. and Subsidiaries | Series 2018A |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | $ 52,000,000 |
Unsecured Debt | Hawaiian Electric Company, Inc. and Subsidiaries | Series 2018B |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 12,500,000 |
Unsecured Debt | Hawaiian Electric Company, Inc. and Subsidiaries | Series 2018C |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 10,500,000 |
Unsecured Debt | Hawaii Electric Light Company, Inc. (HELCO) | Series 2018A |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 9,000,000 |
Unsecured Debt | Hawaii Electric Light Company, Inc. (HELCO) | Series 2018B |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 3,000,000 |
Unsecured Debt | Hawaii Electric Light Company, Inc. (HELCO) | Series 2018C |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 3,000,000 |
Unsecured Debt | Maui Electric | Series 2018A |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 6,500,000 |
Unsecured Debt | Maui Electric | Series 2018B |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | 2,000,000 |
Unsecured Debt | Maui Electric | Series 2018C |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  |  | $ 1,500,000 |
Unsecured Debt | Mauo, LLC |  |  |  |  |
Credit agreement |  |  |  |  |
Aggregate principal amount |  |  | $ 0 |  |
Debt, face amount | $ 50,500,000.0 |  |  |  |
Unsecured Debt | Mauo, LLC | London Interbank Offered Rate (LIBOR) |  |  |  |  |
Credit agreement |  |  |  |  |
Basis spread on variable rate | 1.375% |  |  |  |

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