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Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Pension Benefits |  |
Accumulated other comprehensive loss (pre-tax) |  |
Net actuarial loss (gain) | $ 10.4 | $ 10.6
Prior service credits | 0.0 | 0.0
Total | 10.4 | 10.6
Net regulatory assets (liabilities) |  |
Net actuarial loss (gain) | 1,101.2 | 1,067.7
Prior service costs (credits) | 1.1 | 2.7
Total | 1,102.3 | 1,070.4
OPEB Benefits |  |
Accumulated other comprehensive loss (pre-tax) |  |
Net actuarial loss (gain) | (1.4) | (1.6)
Prior service credits | (0.1) | (0.1)
Total | (1.5) | (1.7)
Net regulatory assets (liabilities) |  |
Net actuarial loss (gain) | (288.7) | (266.6)
Prior service costs (credits) | (78.6) | (88.6)
Total | $ (367.3) | $ (355.2)

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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i-k) -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669646-108580 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + 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 (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 + Details
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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + 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 (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=121641772&loc=SL7669686-108580 + Details
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- Definition The total of net gain (loss), prior service cost (credit), and transition assets (obligations), as well as other items included in net regulatory assets associated with a defined benefit pension or other postretirement plan(s) because they have yet to be recognized as components of net periodic benefit cost. + References No definition available. + Details
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- Definition The net amount of actuarial gains and losses, which has not yet been recognized as components of net periodic benefit cost, and that are recognized as net regulatory assets. + References No definition available. + Details
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- Definition The net amount of prior service cost or credit relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost, and that are recognized as net regulatory assets. + References No definition available. + Details
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