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Employee Benefits - Amounts Not Yet Recognized in Net Periodic Benefit Cost (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Pension Costs |  |
Accumulated other comprehensive loss (pre-tax) |  |
Net actuarial loss (gain) | $ 10.0 | $ 12.0
Prior service credits | 0.0 | 0.0
Total | 10.0 | 12.0
Net regulatory assets |  |
Net actuarial loss (gain) | 1,136.8 | 1,240.7
Prior service costs (credits) | 7.5 | 10.5
Total | 1,144.3 | 1,251.2
Estimated amounts that will be amortized into net periodic benefit cost next year |  |
Net actuarial loss | 92.5 |
Prior service costs (credits) | 2.6 |
Total 2018 – estimated amortization | 95.1 |
OPEB Costs |  |
Accumulated other comprehensive loss (pre-tax) |  |
Net actuarial loss (gain) | (1.0) | (1.0)
Prior service credits | (0.1) | 0.0
Total | (1.1) | (1.0)
Net regulatory assets |  |
Net actuarial loss (gain) | (4.7) | 25.8
Prior service costs (credits) | (111.8) | (87.9)
Total | (116.5) | $ (62.1)
Estimated amounts that will be amortized into net periodic benefit cost next year |  |
Net actuarial loss | 1.3 |
Prior service costs (credits) | (15.3) |
Total 2018 – estimated amortization | $ (14.0) |

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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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + 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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