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Retirement benefits - Components of NPBC (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension benefits |  |  |
Defined benefit plans |  |  |
Service cost | $ 64,906 | $ 60,555 | $ 66,260
Interest cost | 81,185 | 81,549 | 76,960
Expected return on plan assets | (102,745) | (98,559) | (88,554)
Amortization of net prior service (gain) cost | (55) | (57) | 4
Amortization of net actuarial losses | 26,496 | 24,832 | 36,800
Net periodic pension/benefit cost | 69,787 | 68,320 | 91,470
Impact of PUC D&Os | (18,004) | (18,117) | (40,011)
Net periodic pension/benefit cost (adjusted for impact of PUC D&Os) | 51,783 | 50,203 | 51,459
Amounts amortized from AOCI or regulatory assets into net periodic pension benefit cost during 2015 |  |  |
Prior service credit | 0 |  |
Net actuarial loss | 29,600 |  |
Pension benefits | Hawaiian Electric Company, Inc. and Subsidiaries |  |  |
Defined benefit plans |  |  |
Service cost | 63,059 | 58,796 | 64,262
Interest cost | 74,632 | 74,808 | 70,529
Expected return on plan assets | (95,892) | (91,633) | (82,541)
Amortization of net prior service (gain) cost | 8 | 13 | 40
Amortization of net actuarial losses | 24,392 | 22,693 | 33,371
Net periodic pension/benefit cost | 66,199 | 64,677 | 85,661
Impact of PUC D&Os | (18,004) | (18,117) | (40,011)
Net periodic pension/benefit cost (adjusted for impact of PUC D&Os) | 48,195 | 46,560 | 45,650
Amounts amortized from AOCI or regulatory assets into net periodic pension benefit cost during 2015 |  |  |
Prior service credit | 0 |  |
Net actuarial loss | 26,800 |  |
Other benefits |  |  |
Defined benefit plans |  |  |
Service cost | 3,374 | 3,331 | 3,927
Interest cost | 9,453 | 9,670 | 9,011
Expected return on plan assets | (12,326) | (12,273) | (11,664)
Amortization of net prior service (gain) cost | (1,793) | (1,793) | (1,793)
Amortization of net actuarial losses | 1,130 | 804 | 1,796
Net periodic pension/benefit cost | (162) | (261) | 1,277
Impact of PUC D&Os | 1,211 | 1,343 | (240)
Net periodic pension/benefit cost (adjusted for impact of PUC D&Os) | 1,049 | 1,082 | 1,037
Amounts amortized from AOCI or regulatory assets into net periodic pension benefit cost during 2015 |  |  |
Prior service credit | (1,800) |  |
Net actuarial loss | 0 |  |
Other benefits | Hawaiian Electric Company, Inc. and Subsidiaries |  |  |
Defined benefit plans |  |  |
Service cost | 3,353 | 3,284 | 3,870
Interest cost | 9,115 | 9,337 | 8,700
Expected return on plan assets | (12,147) | (12,096) | (11,495)
Amortization of net prior service (gain) cost | (1,804) | (1,803) | (1,804)
Amortization of net actuarial losses | 1,102 | 793 | 1,754
Net periodic pension/benefit cost | (381) | (485) | 1,025
Impact of PUC D&Os | 1,211 | 1,343 | (240)
Net periodic pension/benefit cost (adjusted for impact of PUC D&Os) | 830 | $ 858 | $ 785
Amounts amortized from AOCI or regulatory assets into net periodic pension benefit cost during 2015 |  |  |
Prior service credit | (1,800) |  |
Net actuarial loss | $ 0 |  |

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- Definition Represents the total amount of net periodic benefit cost adjusted for impact of Public Utilities Commission's decisions and orders. + References No definition available. + Details
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- Definition The amount of gains or losses recognized in net periodic benefit cost. + 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation. + 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 (h)(1) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=49177896 + Details
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- Definition An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time. + 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 (h)(2) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + 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 (h) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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