Retirement benefits - Weighted Average Asset Allocation of Defined Benefit Retirement Plans and Weighted Average Assumptions(Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	3.74%	4.26%	4.60%
Rate of compensation increase	3.50%	3.50%	3.50%
Net periodic pension/benefit cost (years ended)
Discount rate	4.26%	4.60%	4.22%
Expected return on plan assets	7.50%	7.75%	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	4.46%	4.80%
Pension benefits \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	73.00%	71.00%
Asset category, Target
Target	70.00%
Target minimum range	65.00%
Target maximum range	75.00%
Pension benefits \| Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	27.00%	29.00%
Asset category, Target
Target	30.00%
Target minimum range	25.00%
Target maximum range	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	3.72%	4.22%	4.57%
Net periodic pension/benefit cost (years ended)
Discount rate	4.22%	4.57%	4.17%
Expected return on plan assets	7.50%	7.75%	7.75%
Other benefits \| Equity securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	73.00%	70.00%
Asset category, Target
Target	70.00%
Target minimum range	65.00%
Target maximum range	75.00%
Other benefits \| Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Actual asset allocation	27.00%	30.00%
Asset category, Target
Target	30.00%
Target minimum range	25.00%
Target maximum range	35.00%

No definition available.

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The interest rate used to adjust for the time value of money for the plan.

+ References

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- Definition

Expected rate of compensation increases (for pay-related plans).

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The interest rate used to adjust for the time value of money.

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Expected Long-Term Rate of Return on Plan Assets
-URI http://asc.fasb.org/extlink&oid=6512046

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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- Definition

Expected rate of compensation increases (for pay-related plans).

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Target allocation of investments of this type to total plan assets presented on a weighted average basis.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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-Publisher FASB
-Name Accounting Standards Codification
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Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis.

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Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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- Definition

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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