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Retirement Plans - Rate and standard deviation assumptions (Details) - Non-contributory defined benefit pension plan | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Rate of Return Assumption (as a percent) | 7.75% | 7.75% | 7.75%
Cash equivalents |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Actual asset allocation (as a percent) | 2.00% |  |
Cash equivalents | Maximum |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Allowable allocation percentages | 35.00% |  |
Cash equivalents | Minimum |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Allowable allocation percentages | 0.00% |  |
Equities | Maximum |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Allowable allocation percentages | 65.00% |  |
Equities | Minimum |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Allowable allocation percentages | 55.00% |  |
Fixed income |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Actual asset allocation (as a percent) | 98.00% |  |
Fixed income | Maximum |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Allowable allocation percentages | 40.00% |  |
Fixed income | Minimum |  |  |
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan |  |  |
Allowable allocation percentages | 20.00% |  |

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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=118255775&loc=d3e1928-114920 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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