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Retirement Benefits Retirement Benefits - Pension and Other Postretirement Benefit Plan Target Asset Allocations (Details) | Dec. 31, 2018 | Dec. 31, 2017
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Pension Plan, Defined Benefit [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Percentage of Plan Assets |  | 100.00% | 100.00%
Pension Plan, Defined Benefit [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target asset allocation percentage |  | 35.00% |
Percentage of Plan Assets |  | 32.00% | 35.00%
Pension Plan, Defined Benefit [Member] | Fixed Income Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target asset allocation percentage |  | 37.00% |
Percentage of Plan Assets |  | 38.00% | 39.00%
Pension Plan, Defined Benefit [Member] | Alternative Investments [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target asset allocation percentage | [1] | 28.00% |
Percentage of Plan Assets | [1] | 30.00% | 26.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Percentage of Plan Assets |  | 100.00% | 100.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Equity Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target asset allocation percentage |  | 47.00% |
Percentage of Plan Assets |  | 44.00% | 47.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Fixed Income Securities [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target asset allocation percentage |  | 28.00% |
Percentage of Plan Assets |  | 28.00% | 28.00%
Other Postretirement Benefit Plan, Defined Benefit [Member] | Alternative Investments [Member] |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |
Target asset allocation percentage | [1] | 25.00% |
Percentage of Plan Assets | [1] | 28.00% | 25.00%
[1] | Alternative investments include private equity, hedge funds, real estate, and private credit

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + 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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- 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. + References No definition available. + Details
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