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Credit agreements - Summary of Debt Instruments (Details) - Unsecured Notes - USD ($) $ in Millions | Jul. 18, 2019 | Jun. 30, 2019 | May 13, 2019
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Series 2019A |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  |  | $ 50.0
Fixed coupon interest rate |  |  | 4.21%
Series 2019A | Hawaiian Electric (parent only) |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  |  | $ 30.0
Series 2019A | HELCO |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  |  | 10.0
Series 2019A | Maui Electric |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  |  | $ 10.0
2004 Junior subordinated deferrable interest debentures redeemed |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 51.5 |
Fixed coupon interest rate |  | 6.50% |
2004 Junior subordinated deferrable interest debentures redeemed | Hawaiian Electric (parent only) |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 31.5 |
2004 Junior subordinated deferrable interest debentures redeemed | HELCO |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | 10.0 |
2004 Junior subordinated deferrable interest debentures redeemed | Maui Electric |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 10.0 |
Refunding Series 2019 Special Purpose Revenue Bonds | Subsequent Event |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | $ 150.0 |  |
Fixed coupon interest rate | 3.20% |  |
Refunding Series 2019 Special Purpose Revenue Bonds | Hawaiian Electric (parent only) | Subsequent Event |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | $ 90.0 |  |
Refunding Series 2019 Special Purpose Revenue Bonds | HELCO | Subsequent Event |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | 60.0 |  |
Series 2009 Special Purpose Revenue Bonds Redeemed | Subsequent Event |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | $ 150.0 |  |
Fixed coupon interest rate | 6.50% |  |
Series 2009 Special Purpose Revenue Bonds Redeemed | Hawaiian Electric (parent only) | Subsequent Event |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | $ 90.0 |  |
Series 2009 Special Purpose Revenue Bonds Redeemed | HELCO | Subsequent Event |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | $ 60.0 |  |

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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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