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Postretirement Health Care and Life Insurance Benefits - Fair Value of Plan Assets in Non-Represented Trust by Asset (Details) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 USD ($) numberOfTrust | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Number of VEBA trusts | numberOfTrust | 2 |  |  |
Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | $ 1.2 | $ 4.1 | $ 6.2 | $ 11.8
Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 33.7 | 34.2 | $ 15.0 |
Voluntary employees beneficiary association, contribution amount to Non-Represented Trust | 10.7 |  |  |
Voluntary employees beneficiary association, contribution amount to Represented Trust | 9.6 |  |  |
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 19.9 | 20.5 |  |
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 13.8 | 13.7 |  |
Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | $ 0.0 | $ 0.0 |  |
Defined Benefit Plan, Equity Securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Asset allocation strategy for the Non-Represented trust | 30.00% | 50.00% |  |
Fixed Income Securities |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Asset allocation strategy for the Non-Represented trust | 70.00% | 50.00% |  |
U.S. equity securities | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | $ 10.5 | $ 13.0 |  |
U.S. equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 10.5 | 13.0 |  |
U.S. equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
U.S. equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
International equity securities | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 2.0 | 4.0 |  |
International equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 2.0 | 4.0 |  |
International equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
International equity securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Corporate bonds | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 9.6 | 9.2 |  |
Corporate bonds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Corporate bonds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 9.6 | 9.2 |  |
Corporate bonds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
U.S. government securities | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 5.2 | 7.1 |  |
U.S. government securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 1.0 | 2.6 |  |
U.S. government securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 4.2 | 4.5 |  |
U.S. government securities | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Cash funds | Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 6.4 | 0.9 |  |
Cash funds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 6.4 | 0.9 |  |
Cash funds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | 0.0 | 0.0 |  |
Cash funds | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 3 |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |
Defined benefit plan, assets | $ 0.0 | $ 0.0 |  |

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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: 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 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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