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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, 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% |  |

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- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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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 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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