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Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Pension Benefits |  |
Accumulated other comprehensive loss (pre-tax) |  |
Net actuarial loss (gain) | $ 10.6 | $ 14.5
Prior service credits | 0.0 | 0.0
Total | 10.6 | 14.5
Net regulatory assets (liabilities) |  |
Net actuarial loss (gain) | 1,067.7 | 1,184.1
Prior service costs (credits) | 2.7 | 4.9
Total | 1,070.4 | 1,189.0
Estimated amounts that will be amortized into net periodic benefit cost next year |  |
Net actuarial loss (gain) | (97.1) |
Prior service costs (credits) | 1.6 |
Total 2020 – estimated amortization | 98.7 |
OPEB Benefits |  |
Accumulated other comprehensive loss (pre-tax) |  |
Net actuarial loss (gain) | (1.6) | (1.6)
Prior service credits | (0.1) | (0.1)
Total | (1.7) | (1.7)
Net regulatory assets (liabilities) |  |
Net actuarial loss (gain) | (266.6) | (133.0)
Prior service costs (credits) | (88.6) | (100.0)
Total | (355.2) | $ (233.0)
Estimated amounts that will be amortized into net periodic benefit cost next year |  |
Net actuarial loss (gain) | 21.5 |
Prior service costs (credits) | (15.0) |
Total 2020 – estimated amortization | $ (36.5) |

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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://fasb.org/us-gaap/role/ref/legacyRef -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=118930883&loc=SL7669686-108580 + Details
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