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Shareholders' equity - Accumulated other comprehensive income (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | $ 2,097,386 | $ 2,066,753
Current period other comprehensive income (loss) | $ (4,589) | $ 554 | (21,187) | 3,909
Ending Balance | 2,132,045 | 2,103,022 | 2,132,045 | 2,103,022
AOCI |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | (41,941) | (33,129)
Current period other comprehensive income (loss) |  |  | (21,187) | 3,909
Ending Balance | (63,128) | (29,220) | (63,128) | (29,220)
Net unrealized gains (losses) on securities |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | (14,951) | (7,931)
Current period other comprehensive income (loss) |  |  | (22,768) | 2,452
Ending Balance | (37,719) | (5,479) | (37,719) | (5,479)
Unrealized gains (losses) on derivatives |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | 0 | (454)
Current period other comprehensive income (loss) |  |  | 0 | 454
Ending Balance | 0 | 0 | 0 | 0
Retirement benefit plans |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | (26,990) | (24,744)
Current period other comprehensive income (loss) |  |  | 1,581 | 1,003
Ending Balance | (25,409) | (23,741) | (25,409) | (23,741)
Hawaiian Electric Company, Inc. and Subsidiaries |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | 1,845,283 | 1,799,787
Current period other comprehensive income (loss) | 28 | 22 | 85 | 521
Ending Balance | 1,876,237 | 1,829,075 | 1,876,237 | 1,829,075
Hawaiian Electric Company, Inc. and Subsidiaries | AOCI |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | (1,219) | (322)
Current period other comprehensive income (loss) |  |  | 85 | 521
Ending Balance | (1,134) | 199 | (1,134) | 199
Hawaiian Electric Company, Inc. and Subsidiaries | Unrealized gains (losses) on derivatives |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | 0 | (454)
Current period other comprehensive income (loss) |  |  | 0 | 454
Ending Balance | 0 | 0 | 0 | 0
Hawaiian Electric Company, Inc. and Subsidiaries | Retirement benefit plans |  |  |  |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax [Abstract] |  |  |  |
Beginning Balance |  |  | (1,219) | 132
Current period other comprehensive income (loss) |  |  | 85 | 67
Ending Balance | $ (1,134) | $ 199 | $ (1,134) | $ 199

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