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Bank segment (HEI only) - Statements of Comprehensive Income (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Condensed Statement of Income Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income for common stock | $ 32,370 | $ 60,073 | $ 38,661 | $ 34,193 | $ 44,634 | $ 127,142 | $ 44,128 | $ 32,352 | $ 165,297 | $ 248,256 | $ 159,877
Net unrealized losses on available-for sale investment securities: |  |  |  |  |  |  |  |  |  |  |
Net unrealized losses on available-for sale investment securities arising during the period, net of tax benefits of $2,886, $3,763 and $1,541 for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | (4,370) | (5,699) | (2,334)
Reclassification adjustment for net realized gains included in net income, net of taxes of nil, $238 and nil for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | 0 | (360) | 0
Retirement benefit plans: |  |  |  |  |  |  |  |  |  |  |
Net gains arising during the period, net of taxes of nil, nil and $59 for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | 65,531 | (43,510) | 5,889
Adjustment for amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits of $812, $566 and $1,011 for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | 15,737 | 14,518 | 22,465
Other comprehensive income (loss), net of taxes |  |  |  |  |  |  |  |  | (1,372) | (6,867) | 1,116
Comprehensive income attributable to Hawaiian Electric Company, Inc. |  |  |  |  |  |  |  |  | 163,925 | 241,389 | 160,993
American Savings Bank (ASB) |  |  |  |  |  |  |  |  |  |  |
Condensed Statement of Income Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net income for common stock |  |  |  |  |  |  |  |  | 66,997 | 57,279 | 54,730
Net unrealized losses on available-for sale investment securities: |  |  |  |  |  |  |  |  |  |  |
Net unrealized losses on available-for sale investment securities arising during the period, net of tax benefits of $2,886, $3,763 and $1,541 for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | (4,370) | (5,699) | (2,334)
Reclassification adjustment for net realized gains included in net income, net of taxes of nil, $238 and nil for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | 0 | (360) | 0
Retirement benefit plans: |  |  |  |  |  |  |  |  |  |  |
Net gains arising during the period, net of taxes of nil, nil and $59 for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | 0 | 0 | 90
Adjustment for amortization of prior service credit and net losses recognized during the period in net periodic benefit cost, net of tax benefits of $812, $566 and $1,011 for 2017, 2016 and 2015, respectively |  |  |  |  |  |  |  |  | 1,231 | 857 | 1,531
Other comprehensive income (loss), net of taxes |  |  |  |  |  |  |  |  | (3,139) | (5,202) | (713)
Comprehensive income attributable to Hawaiian Electric Company, Inc. |  |  |  |  |  |  |  |  | $ 63,858 | $ 52,077 | $ 54,017

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