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Postretirement Benefits Other Than Pensions - Narrative (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2019 USD ($) employees | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Other Postretirement Benefit Plan, Defined Benefit |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Number of retired employees covered | employees | 650 |  |  |
Number of active employees covered | employees | 525 |  |  |
Pre-tax reduction to its accumulated postretirement benefit obligation | $ 0.0 | $ 0.0 | $ (1.3) | $ 11.0
Effect of one percentage point increase on accumulated postretirement benefit obligation | 0.6 | $ 0.8 |  |
Amounts that will be amortized from AOCI in next fiscal Year | 3.3 |  |  |
Estimated future employer contributions in next fiscal year | $ 1.3 |  |  |
Medical Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Health care cost trend rate assumed for next fiscal year | 7.50% | 7.50% |  |
Health care cost trend rate assumed for future years | 4.80% | 4.80% |  |
Prescription Drug Benefits |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Health care cost trend rate assumed for next fiscal year | 10.00% | 10.00% |  |
Health care cost trend rate assumed for future years | 4.80% | 4.80% |  |

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- Definition Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate. + 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 (m) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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