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Retirement benefits - Weighted Average Asset Allocation of Defined Benefit Retirement Plans and Weighted Average Assumptions (Details) | 12 Months Ended
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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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- Definition 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 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition 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. + 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 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost 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 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition 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. + 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 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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