Retirement Benefits - Plan Target and Actual Asset Allocation (Details)	Dec. 31, 2019	Dec. 31, 2018
Equity securities \| All Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Target Mix	25.00%	30.00%
Actual Mix	24.00%	28.00%
Equity securities \| Postretirement Health Care Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Target Mix	28.00%	32.00%
Actual Mix	29.00%	31.00%
Fixed income securities \| All Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Target Mix	56.00%	51.00%
Actual Mix	57.00%	50.00%
Fixed income securities \| Postretirement Health Care Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Target Mix	52.00%	49.00%
Actual Mix	54.00%	48.00%
Cash and other investments \| All Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Target Mix	19.00%	19.00%
Actual Mix	19.00%	22.00%
Cash and other investments \| Postretirement Health Care Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Target Mix	20.00%	19.00%
Actual Mix	17.00%	21.00%

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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

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- Definition

Percentage of actual 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/2009/role/commonPracticeRef
-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

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