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Employee Benefits, Defined Benefit Plans Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
VEBA Postretirement Medical Obligations |  |  |  |
Payment for Other Postretirement Benefits |  | $ 15.7 | $ 20.0 |
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |
Fair value of deferred compensation assets |  | 10.8 |  | $ 9.8
Voluntary Employees' beneficiary Association (VEBA) | Union VEBA |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |
Maximum contribution threshold | $ 12.8 |  |  |
Voluntary Employees' beneficiary Association (VEBA) | Salaried VEBA |  |  |  |
VEBA Postretirement Medical Obligations |  |  |  |
Maximum contribution threshold |  | $ 2.9 |  |
Payment for Other Postretirement Benefits | $ 2.9 |  |  |

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- Definition Carrying amount as of the balance sheet date of assets held under deferred compensation agreements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of payment for other postretirement benefits. Excludes pension benefits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585 + Details
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