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Retirement benefits - Weighted Average Asset Allocation of Defined Benefit Retirement Plans and Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Asset category, Target
Target	100.00%
Benefit obligation
Discount rate	4.31%	3.74%	4.26%
Rate of compensation increase	3.50%	3.50%	3.50%
Net periodic pension/benefit cost (years ended)
Discount rate	3.74%	4.26%	4.60%
Expected return on plan assets	7.50%	7.50%	7.75%
Rate of compensation increase	3.50%	3.50%	3.50%
Pension benefits | American Savings Bank (ASB)
Net periodic pension/benefit cost (years ended)
Expected return on plan assets	3.94%	4.46%	4.80%
Pension benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	69.00%	73.00%
Asset category, Target
Target	70.00%
Pension benefits | Equity securities | Minimum
Asset category, Target
Target	65.00%
Pension benefits | Equity securities | Maximum
Asset category, Target
Target	75.00%
Pension benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	31.00%	27.00%
Asset category, Target
Target	30.00%
Pension benefits | Fixed income securities | Minimum
Asset category, Target
Target	25.00%
Pension benefits | Fixed income securities | Maximum
Asset category, Target
Target	35.00%
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	100.00%	100.00%
Asset category, Target
Target	100.00%
Benefit obligation
Discount rate	4.34%	3.72%	4.22%
Net periodic pension/benefit cost (years ended)
Discount rate	3.72%	4.22%	4.57%
Expected return on plan assets	7.50%	7.50%	7.75%
Other benefits | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	70.00%	73.00%
Asset category, Target
Target	70.00%
Other benefits | Equity securities | Minimum
Asset category, Target
Target	65.00%
Other benefits | Equity securities | Maximum
Asset category, Target
Target	75.00%
Other benefits | Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	30.00%	27.00%
Asset category, Target
Target	30.00%
Other benefits | Fixed income securities | Minimum
Asset category, Target
Target	25.00%
Other benefits | Fixed income securities | Maximum
Asset category, Target
Target	35.00%

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- References

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Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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