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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