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Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Actual percentage | 100.00% | 100.00%
Maximum percentage of equity security value of total plans portfolio value that is allowed | 8.00% |
Maximum percentage of any corporation outstanding shares that is allowed for retirement income plan equity portfolio | 5.00% |
Return Seeking Assets [Member] |  |
Actual percentage | 58.00% | 50.00%
Liability Hedging Assets [Member] |  |
Actual percentage | 42.00% | 50.00%
Minimum [Member] | Return Seeking Assets [Member] |  |
Defined Benefit Plan, Target Allocation Percentage | 0.2 |
Minimum [Member] | Liability Hedging Assets [Member] |  |
Defined Benefit Plan, Target Allocation Percentage | 0.4 |
Maximum [Member] | Return Seeking Assets [Member] |  |
Defined Benefit Plan, Target Allocation Percentage | 0.6 |
Maximum [Member] | Liability Hedging Assets [Member] |  |
Defined Benefit Plan, Target Allocation Percentage | 0.8 |

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- Definition The maximum percentage of outstanding shares of any one corporation that is allowed for the Retirement Income Plan. + References No definition available. + Details
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- Definition The maximum percentage of any individual security value of the total Retirement Income Plan's portfolio value that is allowed. + References No definition available. + Details
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- Definition Description of target allocation percentages, or range of percentages, considering the major categories of plan assets as of the latest statement of financial position presented (on a weighted-average basis for employers with more than one plan) that are pertinent to an understanding of the plan investment policies and strategies. + 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 (d)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: 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=82843861&loc=d3e2446-114920 + Details
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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=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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